Strange electrical trouble

I have and 89 Jeep Cherokee that's had some modifications from a few previous owners. I'm learning about fixing vehicles as I encounter the quirks on this one.

There's a weird issue where the radio shuts off when the headlights are turned on. It immediately turns back on when the headlights are turned off. The issue actually resolved itself for a short while and then started occurring again. The stereo itself is aftermarket.

It wasn't very high on my list of priorities until last week when the turn signals started doing the same thing. Only working when the headlights are off. This has now gone from a convenience issue to a safety issue.

This will be my first foray into electrical issues on the Jeep, so forgive me if I need to ask for clarification. I've got a multimeter and plenty of tools (and wiring) available to fix this issue, I just have no clue where to even start looking.

My first instinct tells me to check the wire on the radio for lights. There should be a wire to turn display on/up when lights are on. Next would be grounds and power feeds. Something is obviously not spliced in right. Pull the stereo and check it over.

check the grounds.

check the grounds in the front marker lamp sockets.

and have a look at the big firewall connector and associated wires to the ignition switch. Pay particular attention to the orange wires at the top of the firewall connector. They tend to get burnt over the years. When they do they can no longer supply enough current to avoid massive voltage drops when a hefty load is applied.
The ignition switch is also suspect. Brown wire going to the connector likes to melty melty.

I would first check the radio harness, like Hesh mentioned the radio acc/ign 12v sounds like its connected to the dim instead of the ign. Most radios has the Yellow permanently fused 12v, the red is 12v only when ign is turned on and the orange is for dimming wire. For troubleshooting purposes you can pull the red wire and jump it to the yellow wire, the radio should come on. You can put your DVM on the lead where the red wire was connected, is should be approx 12v, or at least the same as the center of your cig lighter lead. Turn on the lights and see if it drops. If not then rewire the harness.

Here is a link to your radio wiring http://www.cherokeeforum.com/f2/wiring-diagram-radio-speakers-pwr-antenna-7417/

The red should go to "hot in acc (red)" orange to light(blu), yellow to "hot at all times(pink)"

If this is an aftermarket radio then someone probably used the illumination variable ground wire as the radio ground instead of grounding it properly.

So I can't attest to the bad grounds, yet, as I'm still learning how to properly use my multimeter, but I have a new symptom. Jeep didn't start recently, so I jumped it and checked the current from the battery terminals with the engine running. At the very least, the alternator isn't fully functional (only 12 volts) so I'll update if it fixes the other issues.

when I say "check grounds" what I really mean is go clean all of them.

trust me. It's the first thing you should do.
Clean them all now and save yourself the headaches of doing them one by one when they randomly flake in the future.
Consider replacing the strap that goes to the head as well. after 25 years they all tend to suck.
you should also bond the dash metal under the gauges to the body. There's a handy hole in the drivers kick panel area that was intended for the pedal operated e brake. It's 10mm if I remember. Run an additional ground from the battery to the inner fender, or to the alternator case.

If you want to prove this to yourself, hook your multimeter to measure volts. Put one lead on the battery negative post and the other on the radio ground. Then turn on the lights. The voltage should not change.

I'm still learning about the wiring, but I solved some of the problems. The power cable from the alternator to the rest of the system was either glued or taped to the connector on the alternator side. A total hack job by one of the previous owners. I replaced the cable and charged the battery and now all my lights work normally.

One of my buddies who knows far more about wiring than I do ran a new ground wire for the radio, but now it doesn't turn on at all. I didn't actually have the radio to test the new ground when he did it (it wasn't the primary job of the day) so we didn't know it didn't work till the next day. I'm going to be diving into redoing all the wiring for it next. Whoever installed it stripped and connected literally every single wire running into it, so who knows what may or may not be right.

I hate people that cut the harness.

an adapter is $10, why destroy the chassis harness and punish yorself in the dash when you can just make a plug and play solution on your workbench.

My latest MJ has had the radio harness all hacked up, which means I'll end up hardwiring my install, and I really hate the person that forced me into that.

That was kind of what went through my head when I first saw it. I have learned a lot about wiring just in the last 2 weeks and it seems like whoever had the Jeep last knew enough to make some circuits work but not enough to spend the $30 that would have made them all legitimate with bought parts.