I just bought and installed a AVH-X3500BHS and I got a parking brake bypass off of Amazon as well.

For some reason the Appradio button on the unit is not selectable. A search hinted that it might be the parking brake switch, but the unit plays DVDs fine with the bypass I installed.

Not sure what I am doing wrong. Any ideas?

Remove the bypass and ground the p brake wire and see if that changes anything. If it works you know you have the wrong bypass.

Ill give it a shot, but why would it work for the DVD playback and not the Appradio?

If it like my 1500 you need a special cable to enable apps on it