Electrical nightmare

So my RMW has -SOMETHING- that apparently is causing sequential electrical failures (or maybe they are not related to a single cause but i don't think so).
-Driver door power lock start to "buzz" when engine is on, especially in low (locked) position. I've replaced the actuator in the door and also the relay, with no success.
-Water temperature gauge is below "zero" when cold, and at the second line when hot. It NEVER goes further (it must be in vertical, in the middle of the gauge, when hot).
-Keyless entry works only below few meters from the car. When I bought it, it works at 15-20m distance (yes, i've replaced the batteries, multiple times)
-I've an Android 2DIN stereo, i've installed it just after i've bought the car and it works fine. Now it has some background noise and when i switch in reverse and the rear camera lights on, the image is disturbed.
-A month ago, the fuel gauge starts to work bad: first it don't light the "low fuel" light... the light goes on only when the fuel is about to finish; then, when i put gas to about half the tank, the gauge starts to go over the max and returning continously to the 1/2 sign; yesterday it says i have 1/4 tank and the car stopped in the middle of nowhere without gas.
It seems that it ALL started when I install an ampli and a subwoofer, route the cables and connect the ampli to the radio... I have to install a large positive cable on the battery and the factory clamp don't allow it, so i've replaced it with a copper one and reconnect all the cables. BUT... after some of the above mentioned troubles, i've tried to disconnect the ampli cable (so the whole ampli-sub system is off) and nothing changed.
I think it can be some alternator-related issue, it's possible? I think it because the "buzz" on the power lock happen only with engine running (but i must check the other issues yet)

It sounds like a ground issue. Check all of the grounds first. There are grounds at the bottom of the driver's side A pillar. The engine ground at the front, head ground at the back of the driver's side head, There are other grounds, but I would have to look at the FSM to find the locations.

I installed stainless steel studs in the battery terminals, and use stainless steel nuts so I can attach numerous leads to the battery.

Fred Kiehl wrote:

... There are grounds at the bottom of the driver's side A pillar.

How can I reach it?

Fred Kiehl wrote:

There are other grounds, but I would have to look at the FSM to find the locations.

Sorry, what is the FSM?

"what is the FSM"

The Factory Service Manual. Pretty much invaluable if you are doing your own maintenance.

There are usually some available on Ebay.

Freds advice on grounds sounds good, given the many different items afflicted.

Rev Bob wrote:

The Factory Service Manual. Pretty much invaluable if you are doing your own maintenance.

Oh, OK. Must check, for sure I have the Haynes, don't remember if I have also the FSM, but I think not.
Are the wiring diagrams available online?