I'm installing a Pioneer head unit in my 90 Mustang but I need to know where the red accessory wire needs to be spliced into the old wiring at. When I got the car it was on the orange wire which I understand to be illumination and then the stereo would shut off when the head lights come on.
Also the yellow battery wire is spliced into a green wire and I don't know if that is right either
Any help would be greatly appreciated

 

You can get a multimeter and solve all of your issues. Find a good ground source and put the black lead on it and then use the red to test all of the wires on your car's wiring harness. Different years have different colors for the battery and ACC wires, only way to tell what is what is to check it with a volt meter.