HowTo: Ultimate Smart Trunk Light DIY (lots of pics)

While our Smart Passion is a great car, one thing always bugged my wife: The missing Trunk Light.

Of course a missing Trunk Light doesn’t stop the planet from rotating, I thought this would be a good opportunity to earn those all important brownie points, hence I set out to create the ultimate Trunk Light:
  • Invisible/OEM appearance - Ruining the look of our Smart would be just terrible.
  • Permanent - While some solutions to the problem were simple and cute (the LED Puck light comes to mind), they were not permanent or automatic like an OEM solution.
  • Inexpensive and simple - I wanted something that is inexpensive (<$25) and can be installed in a few hours w/o special equipment.

The install has now been running rattle free for a week by now and the only “flaw” that I couldn’t address is that the trunk light is always set to “automatic mode” regardless of the setting of the center interior light switch.

i.e.
  1. The new trunk light turns on by opening the door or trunk.
  2. And, the trunk light turns off upon starting the ignition or when the timer times-out.
Since the trunk door is typically open when we access the trunk space, this hasn’t been a problem for us. In addition, we prefer the automatic-on setting compared to needing to flip a toggle switch or tap a puck light as needed.

Later I’ll explain the reason the trunk light is always set to the “automatic mode” instead of being controlled using the center light switch.

What I ended up was this:

The light is generated by 4 high-powered LEDs shining through holes in the headliner above the trunk:

Note:This picture was shot from 2 ft off the ground. The individual LEDs are not visible for the standard child or adult.

I tried to document all the steps, so people can follow them and maybe earn some brownie points from their own Sweety ;)

Shopping List:
- 4 white LEDs - I used this site, which sells nice ones in a 5 pack.
- some small gauge wire
- misc wire connectors

Some theory first (warning: science content):
The LEDs have a forward current of 3.7 Volts. Usually an LED requires a current limiting resistor, but since we are purposely underpowering our LEDs (we drive them between 3.0 - 3.5V only) that is not an issue, which makes our life a little easier.

So if we take 4 LEDs in series the 12V of the car will give each LED 3V, which is perfect for us (if you drive them close to spec these LEDs will get hot and we would need additional heatsinking and a proper current source, etc.).

Plan of Attack
The whole project is pretty straight forward. Remove rear headliner, remove side head panels, tap into power from center light, run cables back to rear headliner, drill 4 holes for LEDs, hot glue the wired LEDs in place and put everything back together.

Required Tools:


The only real “special” one is this Torx bit:


The drill bit you need is 6.5 mm. The LEDs are 6.84mm, so a little bit of wiggling of the drill will give you a snug fit.

Here are the individual steps:

Removing the rear headliner with 2 Torx screws (star-shaped):


Pulling the side panels down:




Note: You do not have to remove the sun shade if you have a passion.

Pulling the front headliner a little down to run the cable:



Pull the cable from the back to the front:


Note: Use tape or foam to make sure the cable doesn’t rattle! I used some Gorilla tape the entire run and I’m rattle free.

Next Step is IMPORTANT
Put everything together except for the rear headliner to see if there aren’t any new rattles due to the cable. This can be done very nicely by driving to your local coffee shop:


Note: When you turn your radio off, you’ll be surprised by how many rattles you’ll hear. Make sure nothing comes from the areas you worked on.

Drilling holes for the LEDs:


Note: Make sure to place the holes at the rear end of the plastic to ensure you get light down the center of the trunk and don’t end up with all the light pointing to the front (the plastic is slightly angled towards the front). I started with a small pilot drill from the inside and then used the final diameter drill (6.5 mm) from the outside.

Making space for the cables:


Note: As you can tell I cut slots into the material with my Dremel. In hindsight it would have been better to just drill small holes for the wires.

Soldering the LEDs:


Note: Make sure to not overdo it with the solder. If your solder is too thick, your LED won’t be able to stick through the hole. It takes a little practice to get it right as the attached heatsink of the LED makes it tough to get the solder up to temperature.



Note: Make sure to apply hot glue to the LED pucks and all the wires to keep everything in place.


Note: You have to connect them in series. That means you start on one end with the (-) sign, connect the other end of the LED (+) with the next LED’s (-) and so on.

I also installed connectors for easy removal:


Note: Connect the rear panel, but leave it off, so you can test everything. Make sure to use tape to avoid rattles.

Solder connectors to your center light assembly:


Note: Make sure to tape the solder joins you just made.


Note: It pops out nicely if you go under the light on the right hand side with a screw driver and pry it out.

Your final product should look something like this:


Plug it together and close/reopen the doors to make sure it turns on.
If it doesn’t you should check the LED polarity. LEDs only light up if the polarity is right (i.e. + and - is observed). So check that all your LEDs are wired correctly.

Note: The “flaw” I mentioned will prevent you from making it work with the switch in the center light. I couldn’t figure out a way to solder onto the stainless steel metal holding the bulb, so I had to pick another spot. This leave the trunk light always in “automatic mode” regardless of how you set your center light switch.

While this isn’t a biggie for us, it would have been nice to figure out a way to make it work according to the switch setting. Let me know if you have an idea how to “fix” this.

Hope you enjoyed my short little how to!
There are more pictures on my website.

-Erich

Tags: diy, led, light, mod, trunk

Views: 6045

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Great job and super pics of the project. That's the kind of post that really makes this site enjoyable. Now, how to wire into that bulb holder.
Thanks Ron! Glad that you enjoyed my how to.

For now I'm not going to spend any more cycles on wiring it into the bulb holder. I just got a set of nice HIDs, which I want to get installed once we are back from vacation.
Erich, you did a super job on this project. Clear photo's, nice description of what to do, even suggestions on what not to do....i.e. running the wire through a hole instead of the slot, but this was the first one. I was confused on how you turn it on though. Will it work with the switch in the automatic mode? If you do then all the lights would come on I assume. Again nice idea for us and a really nice post.
Thank you!

Yes, the lights turn on automatically the same way your center light comes on with the automatic setting. If you want to have them both on (like I do) you just need to set your center light on automatic.

There is just no way to permanently turn them off like you can do with the switch in the center light. You open the door and they will come on regardless of your switch setting.
I can live with that. I will put this on my list of things to do and thank you for doing all the "hard" part for us.
That looks great.
My compliments to the chef on an excellent description of what you did complete with detail photos and night shots! Nice how-to write up. Thanks and good job. When can I drop my smart off at your house for the 2nd one?
Thank you all for the kind responses!

@GasHog: No problem dropping off your car! Just make sure to bring enough beer for a "smooth" install ;)
Eric,
I just completed the same mod (thanks for the tutorial). Due to shipping delays, I used a different emitter (a Luxeon Star, part# LCS-010WL, similar to yours) from LC-LED.com
I also opted to drill holes through the veining, rather than cut down through them. Additionally, I grounded in the back, so I only ran a single wire up front, and tapped into the power lines feeding the dome light, rather than the dome light itself. For future info, there are three wires running to the dome: brown (ground), red (middle) = always on, and blue (door-open power). I tapped into the always-on positive, and added a switch in the leading-edge of the rear ceiling panel (dead center - easy to reach from the back, easy to reach from the front seats).
Here are the results:
Boot (trunk) Lighting Mod


and the switch:

Boot (trunk) Lighting Mod


Thanks for the awesome tutorial!
Gary,

Very, very cool!
Seems like you made a few nice changes to it esp. now having the wire colors is great.

Documenting one's steps makes everything a lot slower (I did my HID install in probably 1/4 of the time w/o documentation), so your reply is very much appreciated.
thanks

I will use this on my smart when I get it. I will put it in my other cars too it's a good looking way to add light. Is there light in the foot wells of the smart or does the over head light take care of that?
No footwell lights, and the dome really doesn't do it either. I plan to add those, soon.

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