I have a 88 E350 cutaway (RV) with a 460 I picked up for a steal. It came new with a am/fm cassette, 2 front door speakers and two speakers in the back.

I'll be putting in an am/fm/mp3 player in. Bought a wiring harness adapter online, hooked it up and no power. Fuse is good but I haven't tested with my voltmeter. Someone did do a small hack job wiring in the 2nd stereo it had but it there was no stereo when I got it. There are 4 wires that have tape but are not labeled (2 gray, 1 blue, 1 red) and don't seem to be from the factory harness.

Thanks!

 

Generally, but not always,
Red is usually for power to the radio, and blue and grey were for speakers.
You might try the radio mfgs website for a diagram of that model . Sometimes inside one of the covers on the radio there is a diagram and that might help to determine which wires go where.