Radio wiring problems

So I am trying to install an original radio back into my truck. I found a stereo radio as well as an eight track w radio. I disconnected the current radio and started Tinkering. Anyway I think in my wiring exploration I might’ve shorted something. One of the radios I could get to make noise and the one with the eight track player almost was acting like it wasn’t even getting power. My wiring system has been modified slightly.  the black A+ line Appears to connectthe eight track player if I match original wiring with original wiring. However the power supply for the stereo radio has a light blue with red wire (so did the 8track) and A yellow wire. If I connected “like with like”  it pulled in stations. The yellow wire appears to be added as I can trace this new yellow wire to the cigarette lighter yellow wire where someone spliced it. Both radios appear to have a choke.

Anyway in tinkering I think I accidentally ground the blue or yellow wire  to the dash frame because now neither radio seems to be working.  I’m hoping I didn’t short out my radio .  I checked my fuses and my horn fuse was blown, but it didn’t seem to fix the problem if I switched around some fuses. Can anyone tell me what voltage I should be measuring off the black A+ wire, the yellow wire, and the blue with red wire? Currently I’m not measuring any voltage now between any of these wires and I’m not measuring any voltage between Any individual wire and ground.  My horn is not working nor is my cigarette lighter And  I didn’t appear to have any voltage under the hood going to the horn itself.  What else is  electronically upstream of all these devices that I might have damaged and how do I check that?

Finally to double check as maybe  i’m trying to measure the long thing, I’ve been trying to measure VDC Since this is a battery system and I assumed as direct current and not alternating current.

Yes, it's 12VDC.  Abandon the dash wiring for a moment, and try to use jumper wires from the battery (or any 12V battery) to power the radios.  LB/R is dash backlighting, so it's not necessary.  Radio power should be Y/Bk, and negative (return, ground) should be Bk.  Post lists of the factory dash wires and of the radio wires.  Photos might help.

None of these radios seem to have a ground then related to the power supply? They  both only have two wires coming out of them related to the power and you’re saying one of them is just the back light  (  which in my tests never did seem to light up in the radio either I’m not really sure what I’m looking for ). How does the circuit compete itself? Is the radio body the ground... which might explain why it wasn’t working when I’m holding in my hands  .

The only black wire coming out of the radio itself appears to be the common ground for the speakers.

Radio body should be the ground. There's a bracket that bolts to the radio which then clips to the metal support of the dash. That is the ground.

Check out the below ebay listing. You can see the clip bolted to the back of the radio with one single bolt.

Radio Clip

Ah! So then  it sounds like I didn’t have the radio grounded at all which is obviously a big part of the problem.

Success! So I didn’t do that test while connected to speakers but both the 8 track activated and the radio light turned on, so that’s great. Next step is to figure out where my power went in the dash!

I would suggest grabbing a piece of thick yet pliable slotted flat bar and using that to make a new radio support bracket/ground. Without it that radio is going to bounce around and you want that thing as secure as possible so nothing inside shakes itself loose.

All of the '81 wiring diagrams are here: Electrical/EVTM's/1981 EVTM.  The Fuse Block & Circuit Protection pages tell what fuse supports what functions.  And the Radios pages tell how those are to be connected.

Rather than deal with 20year old fuses I just replaced them all. Radio and 8 track are up and running!its a bummer the PO cut up the face place where the knobs are for the last radio but it actually isn’t too noticeable for now.  The horn however is still not working although at least the relay is clicking again now the the horn and radio circuit is energized again.

Good job.  

On the horn, put a test light at the horn to see if it is getting power.  If not, test at the output of the relay.  Then the input of the relay.