Fuse blew, no tail lights, no instrument lights.

Traced the fault to the RADIO! Disconnect the radio, turn lights on, fuse doesn't blow!

Tested the radio, no good, replaced radio.

Now, new radio on, no problem; lights on, radio on, fuse blows!

Disconnect the radio, all is well!

Clueless at this point!

 

Your radio/stereo is wired up incorrectly. The tail light, front marker lights, and the cluster lights are all on the same circuit, protected by a fuse marked TAIL.

Your radio/stereo should be on a circuit with fuse labeled CIGAR; a modern stereo would have a red wire attached here, and a yellow wire attached to the circuit with fuse labeled ROOM which is always "hot". Both those stereo wires should be protected by a small in-line fuse.

So which fuses are actually blowing?

 

TAIL is the fuse blowing.
That's the mystery, the radio is wired to the original wiring harness and the CIG fuse is fine, the radio works as long as the lights are not switched on!
When I unplug the radio in the back, all is well with the TAIL fuse but if the radio is plugged in AND the lights come on, the TAIL fuse blows. The radio is fine by itself, this is the second radio I installed after the first one died. I originally took out the OEM radio to install the first radio so the harness is correct, I just wired the new radio's harness to the OEM harness.
This thing got me puzzled, I don't want to give up and take it in to the experts!

 

Wire the stereo red wire to the CIGAR fuse, with an inline fuse in that wire.

Wire the stereo yellow wire to the ROOM fuse, with an inline fuse in that wire.

A stereo pulls very few amps, unless you add a big amplifier.

 

Is your radio factory or aftermarket?

Keep the stereo unplugged, replace fuse, turn on lights, find which stereo wire has power from lights circuit. Cut this wire, tape it up. Refit stereo, start car, turn on lights see what happens.

 

The radio now is an Alpine aftermarket.
Thanks for the responses, I'll try the suggestions and will report back.

 

So, consulted the wiring diagram--there is a BL/RD wire for the radio illumination which runs off the TAIL fuse, the only radio related wire that runs off that fuse since the other radio wires are off the CIGAR fuse.
Sure nuff, once I disconnected that pesky illumination wire, all is good!
Thanks to all who responded!

 

Makes sense - that's because back when these trucks were made, radios did not have LEDs, back then they had a small bulb which would be in parallel to the cluster lights through the dimmer wheel, on the same TAIL circuit as the tail lights and front marker lights.

Using the same circuit for cluster and the front markers/tail lights is a "heads up" to the driver that if the cluster lights don't go on when switched, that likely the tail lights are also out, so a warning. This was the case on my 1988 B2200 when I was having headlight switch issue - my first clue was no cluster lights. I later heard a radio show mechanic state this as well, I didn't know that before it happened to me.