Those leather seat heater relays must have something intrinsically weak. The ones in my '95 940 have died as did the ones in my '89 740. I tried re-soldering the 740 ones and that brought them back to life for a while, but the thermostat never did kick in like it did when new. Re-soldering the 940 ones didn't bring them back to life. On the flip side, the non-relayed heaters in my wife's 940 cloth seats don't kick out soon enough and toast your buns.

Unfortunately it's not always easy to find used leather relay units, especially working used ones. In the first place not all that many 700/900's came with leather. You'd mostly be looking for 760/960's and perhaps some late 700 and early 900 turbo and GLE models. Volvo calls these "overheat protected" heated seats. They use a black plastic box relay module (clipped under the seat) that has "LEATHER" printed on the case. These use an internal thermostat and timer circuitry, they do not sense actual seat temperature. I don't believe there were ever any relays used for cloth seats which use the earlier style button thermostat in the lower heater grid -Volvo calls these "normal output" heated seats. Best bet of course is a yard specializing in Volvos, but those yards know list prices and charge accordingly. Used ones are just as likely dead as alive unless the car has been in such a warm area that the heater was rarely used. It's difficult to perform a quick test at the yard unless you pick a real chilly day -even bringing a can of cold spray doesn't work well on these large relays in that plastic case. Some yards are fairly reluctant to give refunds/exchanges on electricals, especially if you've had them a few weeks.

I don't know which part of the circuitry tends to act up -I'm guessing the thermostat opens up at the slightest bit of internal heat. It would be nice if there was a repair for these things so if anyone has sorted it out please post what information you have. Perhaps someone nearby could donate one to Don Foster. The challenge alone will be enough to get him working on it.
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Dave -not to be confused with a real expert, just goofing around at this