I had the same problem with smoke coming from the vents, about two months ago. I done the caps to the front speakers and had used the polarized ones. I had duct tape around them, cause I could find my electrical tape. I also had car fresheners that hook onto the vents. When the car filled up with smoke, I turned off the heater and radio, pulled off the little service doors for the speakers. I didn't turn on the radio for two months, but pulled off the tape and threw away the vent fresheners. I haven't had a problem since and have been listening to the stereo for a few weeks now. I haven't changed the cabin filter yet, so I will pull that out soon and check the outside vents for clogging. I had just assumed that the caps had gotten hot and started melting the tape, but most of the smoke had come from the passenger side behind the glove box. I am glad someone else has had this problem, so i don't feel like such a smuck for a crappy job on my speaker wires, lol.