If you don't find anything, maybe give it to a professional detailer, someone who is accustomed to deep cleaning a car, to see if maybe they can find the source?
Did you ever address it with Hendrick Cadillac back when the car was new? If so, what did they say?
Is the car always closed up in the garage? If so, maybe let it sit with the windows down to "air out", either in the garage or out in the sun on a nice day?
Did you ever address it with Hendrick Cadillac back when the car was new? If so, what did they say?
Is the car always closed up in the garage? If so, maybe let it sit with the windows down to "air out", either in the garage or out in the sun on a nice day?