Car smell

I drove my car (2006 rav4) while the water pump overheated trying to make it home and did not make it. I got the water pump fixed and a couple months later I had a coolant leak from my head gasket into my pistons.the head was resurfaced and new gaskets. I now have a smell coming out of the air vent, smells like exhaust or coolant. The machanic says he can not find the source. I don’t know what to look for or do, I brought it back more than five times. can somebody help.

Sounds like a small leak in the heater core to me.