Missing the old days

[Hickschnick]

I have 2008 Pontiac grand prix I installed an aftermarket stereo in. Since the install the dash, headlights and hud will randomly shut off while driving. Hud will also randomly show a charging system failure but alternator tests fine. The car didn't do any of this before the install. ive been told it could be that Ive installed the wrong harness and interface???? I have no idea. When looking online i see just harnesses, kits w/interfaces and other stuff. When it comes to these newer vehicles I'm lost om what to do. I miss the old days when it was as simple as connecting matching colored wires, plugging it in and you're done. Someone please point me in the right direction. Also the car has steering wheel controls and i added the interface for that. They operate as they should.