As topic states, my heater only works on settings 3 + 4. I heard it was the heater resistor. Where is this located?

 

not sure, but it could also be some of the solder points in the climate control unit. i had to re-solder some of them in my previous car ('91 teg LS) just to get it to work on any speed other than high.

 

Remove the 4 or 5 screws and the draw catches holding in glove box.
Remove glove box. You'll see a big roundish looking thing sticking out from the firewall. That's the blower motor. The resistor is screwed into the top of it.