GM-GMC: chevy heater, chevy z71, first mistake

Question
I own a 1989 chevy Z71 p/u truck. The heat is not very warm on all settings. I drained the coolant and replaced this with Dexcool. Could this be heater core? there is a funny smell from the vents. Or would this be the thermostat being stuck open? Temperature gauge reads 1/4 - I was told this needs to read 1/3 to 1/2.

Answer
The first mistake is puting dexcool in. In order to exchange coolant you have to flush the system many times to change from the green to dexcool or it will not last any longer than the green.

I took dexcool out of my car and put the green back in because dexcool sludges up. I have the tsbs for that. Also the smell may be the heater core but if your heater is working then most likely not.

Since the therm is the cheapest to replace and fix I would check that first.

Did this happen before or after dexcool was put in?