Replying to your PM here as well
The body/chassis of your car is the ground/negative for all/most of your electrical connections. Wires for the power/positive run through the wiring harness for all the different components; radio, lights, motors, etc. There is a grounding strap that goes from your battery's negative post to the car's chassis. If the wire has a bad or corroded connection at the chassis connection or at the battery negative post you will have all kinds of strange things happening.
DC electricity (all cars have DC electricity in the wiring) flows from high potential to low potential. If you have a poor ground connection then electricity looks for another low potential. This could be the positive wire on another device in the circuit. That is probably why your interior lights come on (dimly) when you step on the brake.
You don't say how you are testing the fender for a good ground connection. If you don't have a multimeter then try running a separate wire from the NEGATIVE battery post to the fender. Find a bolt on the fender (maybe in the wheel well area) and connect to that, then try your brakes, turn signal etc. If they work properly then you have a poor ground to the chassis.
