This did it with the original stereo and with the new aftermarket unit. On AM radio with a rather weak signal, you can adjust the heater temp control and make the static worse or better. It sounds like there is an electric motor in use when the temp is adjusted. Are these cable operated or electric? Truck has A/C.

Thanks!!!

 

There are electric blend door actuators for the heating system. Do you get static anywhere else? Strong AM or any FM stations? If it's only weak AM stations, I wouldn't kill myself looking for it, you could be chasing something that isn't broken, but just is, AM is susceptible to static.