I've inherited my dad's old 2014 Mustang 3.7L Premium, and I'm trying to return the audio system back to stock after a sketchy install my dad was fleeced $2500 out of for. I've removed everything to include the aftermarket amplifiers, head unit, dash kit w/ AC controls, and related wiring. I kept the aftermarket speakers and installed one of those generic Android head units. It works perfectly and even has a plug & play wiring harness, a major selling point for me.

To keep it short and sweet, I'm having trouble with the door & rear speakers. When I first start the car, they'll play at a low volume before cutting out, occasionally playing audio again very loudly and sometimes distortedly for a second before cutting out again. The problems become more frequent when the music is set at a higher volume.

Note: I'm not having issues with the OEM 8" subs in the doors.

Equipment Specs:
• Amplifier: OEM Premium/Shaker 500 (370w max) [P/N: DR3T18T806AF]
• Door Speakers: 6.5" Pioneer TS-A1687S (350w max, 80w RMS, 4ohms)
• Door Subs: 8" OEM (Dual voice coil, 60w x 2, 1.2ohm x 2)
• Rear Speakers: 6"x9" Pioneer TS-A6967S (450w max, 90w RMS, 4ohms)

Here's what I've done:
• I've tested the speakers with a multimeter and they seem fine
• I also tested a different pair of speakers but the problem remains
• I've tried a different head unit and wiring harness as well but nothing changed
• I've messed with the sound & EQ settings in the head unit too but that didn't help

Here are discrepancies I've found:
• The speakers were installed in reverse polarity. I removed the spliced wires and bought a proper adapter that plugs into the factory speaker plug (Metra 72-5601)
• 2 pairs of fuses were wired together in the fusebox found behind the passenger side kick panel. I removed the wires and replaced the fuses, 3 of which were blown. After this I noticed small electrical features worked again.
• Again on the passenger side kick panel, there's a connector that connects the dash wiring harness to the body wiring harness. There are a few wires that are spliced for some unknown reason.
• In the driver-side footwell, there's a connector with 3 pins that is hanging. I haven't found what it's supposed to connect to.

Images:
• Passenger-side kick panel fusebox (https://imgur.com/a/tgxdXsm)
• Passenger-side kick panel wiring harness (https://imgur.com/a/8rJIa2h)
• Driver-side hanging connector (https://imgur.com/a/5v1vaca)

Any advice or pointers towards the right direction would be greatly appreciated. I can provide audio clips of the speakers & images of anything if needed.

 

A few shots in the dark:

1. There is an intermittent short somewhere in the front speaker wiring. IIRC the subs and speakers are on one channel, so you'd be looking for rubbing/frayed wiring between the Y split and the speaker. The head unit may cut both R/L if it gets overloaded (but now that I've written it out, this would also cause the subs to cut out, *unless* I am wrong and the subs are separate channels from the mid/high door speakers).
2. You mentioned the speakers check out via multi-meter. I would also check resistance at the harness/head unit end, and compare that to what your aftermarket head unit is capable of.
3. Are you sure these awesome installers didn't install some additional amplifier(s) for those door speakers? If so, that's another potential failure point.

Another thing you might try, in an attempt to limit your search: Disconnect one side and see if the problem disappears/changes. Then do the opposite.
Good luck. It sounds like someone did quite a number on the electrical system