I recently had my timing belt, water pump, and thermostat replaced in my '95 Millenia. it is still overheating when i turn on the heat. I was told it could probably be the thermostat i used which is after-market. Can anyone give me anymore reasons why it's overheating or is this pretty much the cause?

Are you sure you installed the thermostat correctly? The thermostat spring should always be facing the engine block and flat part of the thermostat poiting away from the block toward the radiator.

you might need a radiator flush

Your car should really overheat less when you turn your heater on. This is because your heater core is a miniature radiator and helps dissipate engine heat. In fact, when your car overheats accidentally, it is recommended that you leave it idling for a short while with the heater on.

You may have a clogged radiator (if the thermostat is properly installed and functional). The radiator passage tubes get cloged with with scale and other debris with time. Do you have the proper coolant mixture in your radiator 950/50 coolant water mix)? You should always use distilled water because using ordinary tap water accelerates clogging of radiator tubes with mineral deposits that precipitate out.

I FOUND THAT THE HOUSING FOR THE THERMOSTAT HAS A FACTORY DEFECT IN IT THAT CAUSES IT TO LEAK AND LET AIR INTO THE WATER SYSTEM. THERE HAS NOT BEEN A RECALL ON THIS THOUGH. THE FIX THAT I MADE WAS TO REPLACE THE HOUSING.

the coolant might be low check that, if its less fill it up and then see the difference

This happened to me! Lifted the bonnet on a 626 Diesel while it was running and noticed the top radiator hose was sucked in! The radiator bottle had the correct amount of coolant and the temp gauge was on normal. Undid the radiator cap and the hose went back to normal but the coolant in the radiator was down a bit! There is a small valve in the radiator cap which allows the radiator to suck in coolant if needed from the plastic bottle and if that little valve gets clogged, like mine was, it can't suck it in!Just cleaned it up and good as new!May not be your problem, just seemed like a good post to mention it!!