I agree with the others: once you've established that the heater circuit is opened fully, check that thermostat! If it is missing or stuck open, the engine may not completely warm up. This effect is amplified in Winter, and if your driving is on the freeway. Is the heat guage working? Does it show that the engine is fully hot?

A thermostat / gasket will run you less than $10. A heater core replacement will drive a sane person crazy.

Good luck!

1993 Trooper
"Purple People Eater"