2000 s10 electrical troubles

[awe341max]

I am having some odd electrical issues with my dad's 2000 s10 and I'm not quite sure where to start looking for the problem.

First off there is a constant flickering of all of the interior dash lights including the clock on the radio when the truck is running. This gets worse when the headlights are on and even worse yet when the AC is running as well.
Both the alternator and battery are new and test good.

Second is the brake lights and reverse lights both coming on when you hit the brakes. The reverse lights don't always come on when you hit the brakes though, it is very intermittent. Sometimes it is only one of the reverse lights that comes on, sometimes it is both, and sometimes both come on but one is brighter then the other.

The third thing is the driver side rear turn signal never works when the headlights are on. However the 4 way flashers will always work.
all of the bulbs have been replaced and there was no corrosion on any of the connectors.

The truck did have a trailer wiring harness that has been installed for several years now but I have already removed it hoping that some of these problems were due to a short in that harness. No such luck there.

I am sure that all of these things are related but I'm at a bit of a loss as to exactly where to look to find the problem.

Any help with this would be greatly appreciated as I really don't like my dad driving his truck around without fully functional brake lights and turn signals.

[MT-2500]

On starnge light problems always check all grounds and connections and bulb sockets and bulbs.
MT