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Similar problems here. Looking at the Volvo wiring diagram there seem to be two types of seat thermostats.
The older type is the small button you describe. It sits on top of a tapered spring coil which is inserted in the seat cushion itself. There is no seat heater relay with that system. Indeed if the wires manage to get pulled out then that's going to kill any continuity through the heater grid just as would a broken grid wire. Fortunately the newer seat heater grids aren't known for internal broken wires as much as the earlier ones.
The newer type of thermostat is a combined seat heater relay/control module. It is clipped under the seat between springs. The thermostat for it is less sensitive to seat surface temperature as it is controlled by an internal thermostat and duty cycle timer. The electrical connectors for this system are a positive lock type although with enough effort the wires might pull out of their pins or the pins might slightly pull out of the connector to make a poor contact. This latter system was in my '89 740 and is in my '95 940. If this relay fails and you need a replacement, make sure you check to see that you get the correct type. The one for leather seats clearly says "leather" on the plastic case.
In the case of my 740 with the relay/control module, I had poor or intermittent heating. By opening the relays and Fosterizing (re-soldering) the main points I was able to restore full functionality.
In the case of my 940, re-soldering has not helped either seat. I need to dive under the seats and do a bit more digging with my meter to isolate this problem. As usual I'll pick a warm sunny day to do this soon to discover the thermostat is happily tripped and can't be easily tested. Fortunately I keep a can of spray cold on the shelf.
BTW The manual indicates the thermostats are designed to kick in at +14C and out at +26C. On our other 940 with cloth seats the kickout point is seems quite a bit hotter than this. Talk about your toasted buns.
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-Dave (not to be confused with a real expert, just goofing around at this)
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