jvc unit not working

i have a 1988 toyota pickup with a jvc kd-s630 receiver.  i had it installed years ago and it has been working great.  recently it stopped working.  the only thing it will do is display the time when i hit display and accept and eject a cd.  i took the radio out to look for bad connections and found nothing.  i was ready for an upgrade anyway so i bought a new jvc kd-g310.  i unplugged the harness from the old unit and plugged it into the new unit and the same thing happens, so i figure it must be a bad connection somewhere.  i am in the process of installing a new harness and i have the wiring diagram and all, but my question is what should i be looking for as a cause of the problem?  i know very little about installing car audio so any help is appreciated.

It sounds to me like your 12 volt switch (accesory) wire is not hooked up properly.  this is the wire that will power up the unit when the car is turned on check all your fuses first i would almost guarentee you have a blown fuse.  If they all check out ok get out a meter and see if the red wire on the harness is registering 12 volts with the car running