Well, I've only done the driver seat so far. My car is a 1982 300TD with 324k miles. The car isn't exactly tip-top, but I've done my best to take care of it in the last 10 years, and the PO kept it up nicely. But no amount of diligence could prevent a saggy, squishy, torn, unsupportive driver seat.

I got a pair of seats from a 1986 560SEL, which are the genII w126 seats with full power, heat, and memory. I took the seats with all the control units still mounted to the bottom of them. I cut the wire harness on the bottom of the seats, and took the switches with cut harnesses. I originally wanted to put the switches in the doors, but it didn't look like they'd fit flush anywhere. I ended up putting the switches on the floor right in front of the inner rail, so one has to lean forward and pick up the switch to adjust it (like a manual seat). It's not the most aesthetically pleasing solution, but its elegance lies in its simplicity. No need to run wires through the door jamb, cut door panels, etc. All the wires are color coded for soldering the switch to the seat harness, and I just teed the 12gauge RD/WT and RD/YL to power, and the 12gauge BR wire to a good ground. They will be fused as soon as my fuse holders show up in the mail.

The w126 seat does bolt right into a w123. The holes in the floor pan are in the exact same pattern. I read that one could also detach the rails from the seats and mount a w126 seat onto w123 rails to simplify the wiring (and reduce the weight) but there still needs to be a switch for the recline, and for the headrest (if desired), and of course power and ground. The w123 seats are pretty light for such a large seat, I'd guess 30 lbs each. The w126 seats are massive, I'd guess 80-90 lbs each.

It feels like the w126 seat is higher than the w123 seat. Even when lowered all the way, it won't go as low as the original w123 seat. I kept my w123 seat a little above the lowest height, and now with the w126 seats bottomed out, it's a bit higher than I'm used to but I can acclimate to it. If you have large thighs or are very tall, this might be an issue for you.

I had to use the w123 seat belt rail.

I just found out the w123 seat belt buckle won't work with the w126 receiver, so I either have to take the seat out again and try to put the w123 receiver onto the w126 seat, or else find a way to swap out the buckle on the belt itself. I don't think it will be possible without unstitching the belt end. If anyone knows how to resolve the seat belt issue, I'd love to know!

Oh yeah, and I LOVE this seat! No joke, it makes the whole car feel like it handles and rides better. And of course the fact that it's leather is a plus.

Anyway I know it's been done before, but I searched a few forums extensively, and found little documentation on the swap, difficulties, what needs to be modded, etc. so I just wanted to share my experience.

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'82 300TD
'77 6.9
'75 280S
'74 280
'87 Porsche 944 turbo