Heater fan

make sure you dont have voltage drop in the switch or in the wires.

have you run a straight hot to the fan to see if the problem is in the fan.
that is where id start

this is how you check to make sure the circut is correct.

check the voltage drop across the heater when it is running,

wiht the car off you should be dropping about 12.5, or 14+ running. if you are off by more than a couple tenths of a volt, you are losing it somewhere, either through bad ground or corrosion.

proceed the rest of the circut, if the reading is out of spec.