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Larry --
I can speak about repairing the bun warmers in my '86 240 -- I can't assure you they're identical to the 740. I've heard people mention relays for seat warmers, but I've not faced that yet. (But I sure know how to resolder Bosch relays, though!)
On the 240, there are two heaters ("grids") -- one in the bottom, one in the back. They are wired in series (sorta like Christmas tree lights) so if one fails, the other is also off. There's a connector between them.
There's also a connector from the pair to a harness that disappears under the carpet. When I was troubleshooting, I confirmed that 12 volts was present, and (when unplugged) I found the grid resistance was infinite.
(Note -- I don't have the specs, but you should expect resistance somewhere between 3 to 12 ohms, I'd think. 12 ohms means 1 amp of current which means 12 Watts of heat -- not too much. 6 ohms means 2 amps, or 24 Watts, etc.)
When I checked the resistance of the upper and lower grids, I found the lower grid open. I disassembled the seat and seat bottom and removed the grid. On the 240, the grid only extends from the back up to the "buttons" -- so can be pulled without major surgery.
The grid has a small thermostat, about 1/2" diameter by 3/16" thick, as I recall. You can't miss it. One wire had broken off the connection. (Too many fat a**es bouncing too long in that seat, apparently.) I resoldered it and reassembled it.
A month later I was chasing it again -- the same connection had snapped. Evidently, normal flexing exerted too much force, and broke it. I added a 2" pigtail to relieve the stress, and it has been fine for 2 years.
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