After trying to have my old pioneer HU installed and finding out the face plate had died, I had the original Mitsubishi cd player reinstalled.
Then had a few problems with the installer blowing fuses in my car.

Now the question
Is it possible to fry the accessory wire input but the HU still works off constant power?
or what else could be the problem
I've tested the accessory wire and it has power in it.

Are you talking about the accessory power from the car loom, or from the headunit? If its from the headunit I'd say your pretty buggered, need to get a new one. You can wire accessories and constant up the constant from the car, but the headunit won't turn on and off with the car, which will probably run your battery flat one day.

The accessory point in the back of the HU

The accessory point in the back of the HU
I'd say its pretty well messed up then, someone is free to correct me if I'm wrong, but the power the headunit uses to operate actually travels through that accessory power connection. Might need to get another after market headunit to wire up.

If you don't mind me asking what model Pioneer did you have it in? I've got an old Pioneer DEH-2750 lying around that was in my car originally when I bought it, your welcome to make an offer on that if you just want something that basically plays radio and cd. Its in the FS section :)