It is s 2004 coupe. It is not the seats and the smell seems most noticeable in the rear. I cannot smell it in the engine bay. It appears to come and go. I went under the vehicle and checked everything and there are no wires or cables touching and everything is clean underneath. The seats were all off and there is nothing under them that would even burn or melt. So the smell is either an electrical component or wire (connection) or possibly the alternator. Even though the smell is more towards the rear and inside I'm not ruling it out.

Really hard to tell what it is since nothing is failing. I am going to monitor the voltage just to see if it is within range while driving. It could even be the aftermarket radio that was installed years ago....

Is there an external voltage regulator or is it built into the alternator??

Thanks