[Marc Bixen]

It may be coming from the right heat exchanger. If the heater box is closed, the air from the heat exchanger is dumped under the car by the right axle. Smoke from there can come through the gap between the suspension crossmember, and the rear seat pan. Take a look further forward and see if the smoke is actually coming from the area of the RR upper shock mount, forward of the charcoal canister.