When you say smell gets worse when you turn up thermostat temp, I'm assuming you mean the climate control. Sounds like a leak in the heater core which would explain the increase in smell when more hot coolant is sent there when the thermostat is turned up. Check the floorboard carpet to see if it is damp. If it is, you have a bad leak, if it's not, you have a very small leak or no leak at all and some other problem. It is something that you can replace though it is pretty involved. This is just my best shot at your problem and you will probably get better advice from someone on this board more knowledgable that I but check those floor mats first.

Jim