I installed an aftermarket stereo in a 2006 F550 and the speedometer stopped working. I used a Ford adapter to plug into the existing wiring harness. The stero works as expected, but the spedo stopped. Unplugged the stereo and the speedo works again. Any ideas as to where I messed up?

 

The only thing I can think of is that the factory head unit was connected to the speedo for that feature where you speed up the volume goes up or down when you slow down, and somehow with the aftermarket head unit and wire harness takes that out and soemhow messing up the speedo. You might want to go to your local stereo shop and ask. There might be a way to bypass, possibly by hooking 2 wires together. An "IN" to the stereo and "OUT" from the stereo. That is if the guys are nice enough to help. I have been to one around here and they are dicks because I wasn't buying something on that day, but have bought from them before. Never again would I now.

That is all I could think of, but hopefully get you in the right direction to resolve your issue.