Oh my
Ok… depending on clarity of the amp is if it should have the MUCH LARGER ground wire return directly to the battery and have a noise suppressor in line. You should have a large connection point for the ground which uses the same size cable as the power going to it. A common trick is a grommet in the body where the large ground wire goes to the frame where it has been sanded clean and connected then sealed afterwards. Then on the front of the frame near the battery, same sanded connection for large ground cable up to battery negative. A rusty frame negates all that.
The blue wire is usually a remote power on from the head unit to the amp so the amp doesn’t turn on and consume power until that is powered up via key on/ head unit on. They probably cut off the excess blue wire and used it for the small signal ground. Most installers just run a jumper from the small ground to the large ground with a diode to have no noise feed back in.
Definitely check your amp is not powered up all the time- that could be (or other wiring issue with the sound system) the root of the electrical problems you been fighting.
Never run power wires along side the signal wires (sound wires wether before or after any component like amp, crossover, eq, etc. wherever they do have to cross,
Make them cross at a 90° angle like a road intersection.
ALL the wires should be kept as short as possible. Longer wires simply work the component more where you loose sound quality and volume along with picking up more background noise.
