I may not have had a turn-signal stereo (that's classic), but I did have two "mystery wires" dangling below the dash. They were the same color so naturally I thought somebody cut the wire by mistake. Touching them together fired the engine up! I just about put my rig through my fence in the process - as I had left the tranny in gear. Fortunately my reflexes were quick that day...
Let's see - I also had about 4 different trailer wiring harnesses (that were all functional), extra brake lights mounted behind the lenses (wired seperately, via one of the trailer wiring harnesses), and a totally bizarre alarm system (including power lock actuators that were not hooked up to anything - they just clunked pointlessly inside the doors).
Last but not least, sound deadening was provided in the form of composite roofing shingles, glued gravel side down to the entire sheet metal floor of my rig. Covering that was two layers of carpet pad felt, with a rubber floor covering up front and carpet in the back. I will say it was the quietest Bronco interior I'd come across up to that point!
Under the shingles was some rust in the front pans. All has been repaired.