(Girl who wrote a letter complaining that the show sucked and that she had to listen because her parents did in the car... they had her on and she'd periodically call in for years)
My all-time favorite was the episode where someone called in and said that their vehicle would run pretty well but with lots of vibration for 8 minutes, then the engine would cut and refuse to start up again. It turned out to be someone onboard the Space Shuttle. https://www.youtube.com/watch?v=moAqzM4ptm8