Car wiring might be shot? unique situation - please help

Hello,

2004 Chevy Impala LS. Two years ago I had a friend install new speakers and head unit in the car (stupid in hindsight, I know). It seems like he took some shortcuts because the following things have occurred ever since:

1. Radio is not directly linked as it was before - meaning I must manually turn on/off the radio every time I get into the car, it doesn't do it on its own as it should

2. The steering wheel radio controls (volume, etc.) do not work

3. The car's "computer" or whatever seems to be glitchy (the "low tire pressure light" consistently stays on)

All of these are things I can live with. I also plan on keeping the car so resale value is probably reduced by this but that doesn't matter. I hope to someday add on to the sound system (maybe an amp and two new door speakers, new headunit, nothing crazy).

With all that said, are these problems that I should address? I'm looking for more info surrounding price, how easy it is (DIY?), and necessity...like I said, I can live with the changes but I would like to know if it inhibits any further changes to the car.

I was with him the entire time he replaced the two back speakers and headunit so he wasn't acting subordinately when he did this, it was merely the difference between an amateur and professional. My best guess to how he messed up was some connection in the door got skipped over or something.

Sorry if there is a lack of information, I have no real experience with cars or wiring...any advice is greatly appreciated.

Thanks.

EDIT: I was considering bringing to a professional to get their opinion, but we all know that they recommend I get the work done (especially at their shop).

EDIT 2: Cigarette lighters in the car also do not work - no power.