I drive a 2006 VW New Beetle TDI and I would be very happy with my car if it weren’t for the stink that started about a year ago. It’s inside the car and I have no clue what it could be. Nobody spilled anything, we didn’t transport anything or anybody smelly, and from looking around the interior it doesn’t look like a moisture problem (I’m in Austin, TX where it never rains anyway). It’s hard to describe the smell but it’s a little like a mix of melting plastic, burning hair, and dank basement. The dealership “flushed” and cleaned the AC thinking it could be mold but it didn’t make a difference. I’m dreading getting into the car, especially if it has been standing for a day or two and I’m seriously considering selling it (need to research the strongest car deodorizers first).
See if the exhaust system has anything on it, like a plastic bag or other foreign stuff, and make sure it is clear from everything it should be clear from. Also check that there are no fluid leaks, oil, trans etc.
I think the smell is coming from your heater box. I once had a lady’s car come in on the hook. She had been on her way to work and turned her fan on and it started to smoke. I found that mice had been storing paper, plastic and dog food in there. When she turned on the fan, the resistor that in there started to burn this stuff.
You are in Texas so I bet you have had the AC on a lot. That resistor is a bare wire and it gets hot. It would explain the burning hair and plastic smell. The dank basement smell could be a dead mouse. The fan motor and resistor will have to remove to check for this. If a good detailing don’t get rid of it, you will have to find someone with a ozone machine.