Burning smell through vents

Recently my 1995 Aurora developed an acidic (maybe burning plastic; maybe part exhaust) smell blowing through the ventilation ducts after the engine is warmed up. It only does this when the car is stopped with the engine running.

I took it to the Cadillac dealer who works on Aurora's and they could not diagnose the problem.

It may be coincidence, but it seemed to start not long after the spark plugs and wires were changed.

Any ideas? I did partially check the message boards, but nothing similar was apparent.

Do a search with" odor from heater"