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Don Foster (near Cape Cod) is one of the all time great and long serving contributors to the BrickBoard. His fondness is 240's (he has a few in various states of restoration and cannibalization), but his wife has a 740 so he occasionally tinkers in that realm. You'll see all kinds of replies and tips from him in the 700-900 FAQ as well as the archives. One of his most famous fixes is resurrecting dead Bosch relays (especially the troublesome 240 fuel pump relays) by simply re-soldering the internal main board connections. Either he enjoys doing Volvo and Bosch out of overpriced replacement parts or he gets royalties on every Weller soldering gun sold.
In any case, Don loves a challenge. If someone sent Don one or two dead seat heater relays I'm sure he'd tinker with them in his spare time. I'll send him an e-mail regarding this thread. If you're interested, please contact him through this e-mail link to make arrangements.
As for your rheostat idea, I kinda like it, but I'd want to put one of the trustworthy button thermostats into the seat heater grid the way the cloth seats have. There are a couple of reasons for doing this, one is comfort by having the heater system automatically shut down progressively as the car warms up, the other is safety. I've seen references to the occasional seat catching fire as being the reason these thermal relays are now used. Presumably the problem was only with leather seats, either the foam is different or the heat builds up behind the non-vented leather. Even with the thermal relays I've seen seats that look pretty scorched where the relay mounts. Your current limiting resister is a good idea to help prevent this, but on cold days everyone wants maximum initial heat so the tendency might be to use a low resistor and to leave the rheostat wide open while the car warms up in the driveway. This might become a recipe for disaster so it's not something you really want to do by trial and error. BTW, way back when in the 140 days there was a weight sensor as well as a thermostat in the seat. As people wanted a warm seat while their car was idling in the driveway I'm sure people put a heavy block on the seat to fool the switch.
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Dave -not to be confused with a real expert, just goofing around at this
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