Aftermarket remote start. I had a Viper remote start system on mine, and that's where they mounted it, and it had the small microswitch too. I also had a 2003...
I had that problem with a 1st gen ST. Had to replace the rear driveshaft, and the vibration went away. Maybe the cruise at 80mph threw a driveshaft balancing weight? Not that cruising at 80 should cause that, but something more to check.