Finally found the source of that weird shape shifting smell. The carpet padding and the floor is drenched, evenly across the entire cab. And yes, it's been raining cats and dogs, and yes, the rig is parked out doors.
Do I look to door and window seals? Do I look to firewall?
The sheet metal is basically cancer free, so I'm not expecting to find surprise rust outs, but would be good to know what rots most often.
Any help?