Display on kdc-mp638u not working

These units are pretty cut, and dry.. Basicly the only thing you can check for yourself is connection. If the connection between the face plate, and the deck is good, and tight then you have one of 2 problems.
1) Bad face plate. Fix= $90-$130

2) Bad connection to the board on the radio Fix $60-$120

There is however a VERY off chance the processor is jumbled up, and wont cost you a dime to do a cold reset, so give it a shot.

To reset this unit simply remove the face plate, and locate the rest button. It is very small, but easy to find. Just use an ink pen or something of similar size to push the button.