I'm kind of at a loss here. Last month, I sent my 2005 Monte Carlo to the mechanic due to all the electronics suddenly stopping while I was inside the car, and my sunroof being stuck open. I installed a new battery a couple of months ago and just recently tested it with a multimeter. It seems to be fine. The same issue has returned, unfortunately, the mechanic charged me 700 without telling me the problem, and they told me that the cooler had a leak. I'm not sure what the issue could be. This is my first car, and while I'm not super experienced with fixing vehicles, I would prefer to do it myself instead of incurring more mechanic bills. The car was sitting for a while when I got it back from the mechanic, but I doubt that has anything to do with the issue. I used an OBD and I didn't get any codes, and I tested the alternator with my multimeter (when the car was able to run), and the power draw under load seemed sufficient, so I truly have no clue what's going on. If anyone could help me that would be amazing

 

also wanted to say that, for some reason the amp wires in the trunk are cut and kind of just left out, I'm not sure if that's causing my issue or not

 

Just as a fyi, you cant truly test a battery with just a multimeter. A load tester is needed to verify a battery is good. Its not at all uncommon to show full voltage under no load then have a whole cell or two drop out when you put a heavy draw on it.

So it had the issue, they fixed it, then the issue came back?

You say the elecontronics of the car arent working - what exactly are you referring to? Are we saying the car is electrically 100% dead? Or are there specific systems not powering up? IE can you turn on headlights, interior lights, etc, do brake lights work - or is the entire car dead.

Either way, the strategy is the same - you take your multimeter across the power + ground of the non functioning circuit, then trace either forward from the start or backwards from the end until the condition changes. That isolates the probelmatic section.

But if the electrical system is 100% down, then this helps you zone in on the initial connections - verifying all of the primary leads on the battery - power and ground are connected in the right spots with a good corrosion free connection.

I would certainly look at this first. Any aftermarket changes (especially those that look improperly done) should be resolved first.

Should be 2 large wires and 1 small one. Youll want to trace the large power wire back under the hood and completely disconnect it. It should have its own fuse up there, but Id take it off at the source to be 100% safe.

Then there should be a small power wire that comes on with the ignition. Id follow that back to its source (typically either under the rear deck or behind the radio, but they could've run it to a fuse box too. Disconnect the other end of that also.

3rd wire (the other large one) is a ground and shouldn't be an issue.