91 S-10 Heater

If you don't turn the heat on, get it on the freeway and let it warm up, it's fine.
Can't get any heat when driving in town because the heater fuse keeps blowing.
Any suggestions?

Is the fuse the correct amperage? Should say in the owners manual, mine is a 20A. Did you wire anything on that's using power from the blower circuit? Does it blow with the fan on low? If it does, it's probably a problem with the fan motor. Remove the electrical connector from the motor and turn the switch on. If the fuse doesn't blow, the problem is not the motor. If it does blow, you've probably got a shorted wire between the switch and the resistors in the blower plenum.

If it doesn't blow with the fan disconnected, hook power up to the fan with jumper leads. Leave them on just long enough to see if the motor starts or not. If it doesn't turn, replace the motor.