Presently getting close to testing a retrofit in a 190E 16 valve. Time involved is about 30 hours, cost at about $225. It is a hybred system with elements from a Volvo 240DL, thermostats from a Volvo 850, switches from a 560SEL and a relay set from J B Saunders. It is a single function system which will turn on or off the back and bottom at one time. The 560 switch is a control swith only operating a ground and is a daul function when used with a MB controler ( I do not know the P/N or cost ). I have 'breadboard' engineered a relay pak which consists of one double pole power relay and one control relay per seat.
The tear down of the seats is probably half of the job. It is a slow go to get started. Ratcheting cargo straps, picks, hooks and screwdrivers are required for this. I also drilled holes in the seat rails for a 1/4 inch extension in order to remove the aft seat bolts without some special removal tool.
The MB 560SEL switches and pins were new and purchased through Phil. I will get the P/N's to you later.
BTW, My lovely wife considers seat heaters to be a safety feature, claiming that it is impossible to hang on to the steering wheel if she is shivering!
Here is a possible new element source from IPDUSA.
http://www.ipdusa.com/SearchResults.aspx
I have searched Ebay for a considerable time for any used MB seat heater parts and have found nothing. If you desire to put in genuine MB, give Phil a call for parts. My only design criteria was to have period ( '87 ) type switches that will be installed in the shifter switch panel.