1996 rotten smell from A/C help please

I have a 1996 Tahoe. When the heat is turned on there is a rotten smell (dead rat, puke, pig s--t, rotten meat, limburg cheese, ECT) coming from the air ducks. This smell have been on and off for three months now. It seams to be every other week or so. Sometimes I will use the heater for a week and theirs no smell. I live in San Diego and have had the A/C on some days as well, but unable to determine if it might be a factor in the smell. The fan alone has never smelled. Is there a filter in the heating of the Tahoe and if where? Where would I look to see if there might be a rat that got in the A/C duck system and is dead? You would think if it was a rat that it would smell all the time when I run the A/C, with heat, whit Air or just the fan. Any ideas would be helpful. Thanks Dave

That has happened to me and my son describes it as smelling like A__. It is probably caused by clogged drain tubes in the heater/AC air box. The smell is rotting "stuff" from the condensate moisture that collects and needs to drain out the bottom. The condensate is from the humidity that is being taken out if the air when the AC compresser is on, it acts as a dehumidifier just like in your huose.