I changed the heater element on my drivers seat because it had gone pop.
I'm sure on a mk4 though its joined to the leather so they are really expensive. First thing to do is check the relay (normally under the seat) if your passenger seat works then swap the relays.
If that don't work, next thing i did is take both seats out and sit the drivers seat where the passenger seat goes. If it don't work now then you know its the seat not the wiring or switch. I don't know how to strip a mk4 seat but i can imagine it being more difficult than my mk3 Recaro was lol.
I love my heated seats this time of year