Front door speakers cutting out (have OEM Nav non-Bose installed) Where to start?!

[CliffHux]

I just bought this vehicle and it has an OEM non-Bose Nav unit installed. My front door speakers usually don't work, but at times will. It's not a constant cut in and cut out, it's one or the other for a long period of time. Loose ground maybe? Where to look first? Behind head unit? Behind Door panel? Thanks!

 

[Andrew99Tahoe]

When do you notice it happening? When going over a bump, or just randomly?

 

[CliffHux]

It's never while I'm in the vehicle. Sometimes when I start it they work and they'll work the whole time. Then sometimes when I start it they don't work, and don't begin to work.