I have a 2001 Oldsmobile Bravada, relatively in good condition, however my heater is not pumping out good heat as before so I had the thermostat replaced. It worked well for a few weeks, now the heat is not as good.
I've checked my antifreeze coolant, and its fine, therefore there's no leak. Would it be my heating coils and if so where should I look (under dash board, on the right hand side or under the engine hood)
Also my service light is now on.

Please advise