Only thing you can check yourself is the alternator voltage.
connect the battery terminals to a multimeter set in "Voltage mode".
You should see something around 12V with engine off.
When engine is on you will see the voltage jumping to around 13.5 at revvs and around 12.9 at idle. If there are any wild fluctuations then the rectifier place is kaput.
If the steady voltage is below 12.5 at any time or jumps too high its your regulator.
If your service guy says wiring issue, let him tell you what wiring issue is there. You can go to a good installer to check the HU wiring.
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