I can't check right now as my car is in the shop for some front end work but I believe Mike Tangas is correct about the rear seat - it only works if the doors are closed. I know mine was also very problematic until I replaced one of the switches. If you need parts that Princeton MB doesn't have or seem expensive, I've found very good luck in working with Rusty Cullens at ***************. His prices are very good and he'll work with you on the phone to determine what you need - been working with him this morning on control arm bushings myself.
As for the shaky idle, have you noticed any other performance changes? Do you track your gas mileage? While I gather from other posts here that the OVP and idle control valve can cause idle problems - don't think I'm suffering from either of these myself - if you're also seeing severly reduced fuel economy and general performance problems, the O2 sensor might be the culprit.
Good luck with getting things straightened out and welcome to the club. As a relatively new W126 owner myself, I know that it can be a bit unnerving getting the car up to snuff but boy is it worth it