My 1990 Volvo 240 DL's seat heaters haven't been working since I bought the car a few months back. Fixing heated seats sounds like a pain but it's really not that bad. And it's nice to have a warm tuckus on cold winter commutes.
Here's the wiring diagram (click to enlarge). Note my adjustments: the ground coming out of the rear element _does_ flow to the ground, but it's easier to think of it as coming back to the hot wire. There's a 2-way plastic connector that facilitates this.
If your heated-seat indicator bulb illuminates, it means the fuse and wiring up to that point is good. So far so good. I checked for continuity on the wire going into and out of the seat: no good. You can now either open up the seats in the car (warm day) or remove the seat and work on it inside (cold day). Removing the seat is simple and is literally just 4 bolts on the corners. Now the seat cover must come off. Looking at the seat from the back, pop the retaining wire out of its hole and the bottom seat cover will detach in the rear.
Now, recline the back all the way and feed the bottom sear cover up and out. If you want to remove the entire bottom cover (or the back cover) you'll need to cut all the metal hog clips. You can re attach these with zip ties later on....but it's a pain. 90% of problems can be solved without going through this hassle. Reach under the bottom seat cover and pull out the plastic frame. Be careful not to break the 2 connecting wire. The frame is just a molded plastic part with a few meters of wire running through it. It should look like this.
Chances are the non-continuity problem is either (a) a broken wire or (b) a bad thermostat. Actually, both my seats weren't working and one was a broken wire and the other a thermostat. The thermostat allows you to keep your heated seats on during long rides and not burn through the wiring. Basically, when it gets too hot it interrupts the circuit, and when it cools again it allows current to flow. A lot of the time the thermostat goes bad, interrupting the circuit permanently. Check for continuity on either side of the thermostat.
If you have continuity, look for a bad wire and simply reconnect. If no continuity, you need to replace or jump your thermostat. I think you can get a new thermostat for $25 or so online. But I figure you don't really need the thermostat.... just monitor the temperature and switch off the heater when necessary. Your bottom will tell you when. If you think like I do, just grab a couple cm of wire and solder across the thermostat terminals.
Now, you can also buy new heater pads. You can find the bottom and back for less than $100 online. They won't be moulded to the seat foam, but I'm sure you can make it work. If you chose to solder the thermostat terminals instead, just cover with some electrical tape and put it all back together.
Reassembly is the opposite of disassembly :) And MAKE SURE you monitor the temperature of the seats when they are switched on!! Don't leave the switches on when no one is sitting in the chair. You can, however, leave the switches on when you turn the car off...the hot wire is only live when the ignition is on.