this is just for basic troubleshooting, check for voltage at the fuse. im pretty sure the fuse is before the switch not for sure though. if there is voltage at the fuse move to the switch, check voltage at both sides of the switch, should be on only one side with the switch in off position and both sides with the switch on. if only on 1 side while on the switch is bad. if the switch is working(voltage on both sides), you'll have to unplug the fuse and do a continuity check on the seat heater. you unplug the fuse because you dont want voltage applied while in ohms, find the seat heater connector and unplug, measure continuity in ohms through the pins if you get some resistance reading then the seat heater is probably ok. you probably have loose or broken connection between seat and switch. you should have some working knowledge of meters when using in the voltage setting improper use could fry some wiring if shorted the wrong way, then you would be looking at further problems, so make sure if you don't feel confident in using a meter get help.
good luck hope this little bit of info will help.