The engine idle will fluctuate from an intake leak/or vacuum leak, or from a faulty idle air control solenoid (IAC). The IAC has voltage supplied to it when the key is on, so when the key is off it is reseated by a spring, this may explain the second concern you had. The interior lights are turned on by your door ajar switch(s). If the door latches haven't been lubricated regularly then they will stick. This will keep your light on. Spray some WD-40 or other lubricant into the door latches and see if helps. The thermostat is in a housing on the drivers side of the engine. Follow the upper rad hose from the rad to the engine. At the end where the rad hose meets the engine is where the thermostat is. The housing is somewhat round and should have two or three bolts attaching it to the engine. Why do you need to know? Do you have a heating problem?
