I had an issue with the driver's side buckle malfunctioning. The belt would not snap into the buckle. Boris at Smart Madness suggested I call smart USA to see about them taking care of the repair. smart USA got on the case right away and took care of it with my dealer Smart Center Beverly Hills. One more reason to be wild about owning my smart cabrio!
I'm in year 9 of my smart ownership so my Passion is well out of warranty. I've managed to maintain the smart myself for the entire run. I take pride in doing things correctly. I've had a few things fail that are known problems and I've corrected them; the most notable being the ABS sensor ring which requires axle and hub removal to replace with a part I got from England at that time. I've done both sides. The dealer will charge upwards of $800 for this and I did it for under $20 but that is because the dealer replaces the entire drive assembly. I offer this only as background.
My driver's side seatbelt buckle (the female receiver on the console side) has been getting increasingly wonky and over Christmas week it would not latch at all. I spent a fair amount of time diagnosing that it was indeed broken internally. I went looking for a replacement buckle and found many available on eBay and Amazon. I opted for the factory original SMART buckle for about $70 on Amazon which was good as it came with the connector pigtail to hook on the wire harness under the seat.
Changing the buckle was not overly difficult but routing and setting the replacement pigtail wire harness was a big more challenging.
Not sure what the correct procedure is but what I did was to pull the seat forward and kneel on the ground outside to use a ratchet box head wrench to remove the buckle from the seat frame. At this point, you could just cut the wire and replace the buckle but I wanted to keep the wiring correct so then I had use a torx bit on my ratchet wrench to remove the two right side seat back frame support bolts with Torx heads. The wiring harness passes through a small rubber grommet and you have to thread the large connector through that small opening. The new buckle came with the grommet mounted on the wire so once I had loosened the seat back I was able to tip the seat back frame to the left and then remove the old wire and grommet and threat the new one in. After that it was pretty straight forward to simply plugging the new blue connector into the harness and returning it to its mounting bracket under the seat. It took me about an hour but 45 minutes of that time was figuring out how do the job. If I have to this again, it will take 20 minutes or less now knowing how to approach the problem.