I purchased a 1990 mazda miata and when I purchased it the display on an aftermarket tape deck didn't work. So I thought it might just be the tape deck. So I removed a cd player from another car that is brand new and tried it in my Miata and the same thing happened the stereo worked and would turn on take the cd and eject it, but it would not show any display. I ruled out that it was the stereo and believe it could be a wire in the harness or something like that. Hopefully someone can help me out.

 

Wire that is Blue with Red stripe should have voltageall the time thru the 10 amap RADIO fuse.
Wire that is Blue with Black strip should only have voltage when key is in ACC or RUN position thru the 15 amp CIGAR fuse.
Wire that is Red with Black stripe should only have voltage when the lights are turned on.
All of the rest of the wires go to speakers.