I can't imagine what was wrong with it. Everything under there seems in order (not like I've ever seen the underside of these seats before). I'll look around for frayed wires or something like that. But as my brother in law was showing me, there's a little red thing on the circuit board which is supposed to protect against surges. He said that if one of the relays fails, then it could short the board. I've read a pretty scathing complaint about how this sort of thing is common in the 850/70s seats, so I'm not all that surprised.

As for the parts, I've found a place that will sell me the entire mechanicals from a v90 seat (airbags, tracks, motors, memory box, and that control unit for the roller motors) for $70 plus shipping. So I'm just gonna buy the whole thing and throw it under my tool bench for when the motor on one side dies. And if my previous luck with his car is any indication, that should be in about 2 months.