I was recently driving my 1986 300E with the heater on, fan at low speed, and temp setting about 70F. After about 5 minutes, I smelled what seemed to be burning electrical insulation. I turned the thermostat to full cold, but the smell continued. I turned off the fan, and the smell went away. It seemed that that the smell was coming from the duct work. There were no blown fuses, and no obvious problems from the engine compartment. Any suggestions as to where to start looking. The fan motor wasn't noisy or running slow. Any electrical components or harness issues you may have experienced? Thanks in advance.

I have seen the fan motor cause this as well as the wiper motor if there is a binding in the wiper assy.