Voltage changes 12 to 14 volts constantly

I have found using a sealed beam headlight is the best way to test circuits. It puts that circuit your testing under a load.
Get a two wire headlight and run one wire to the battery positive and the other with a t-pin and check grounds right at the computer. If the ground is good you will have a nice bright light.
Do the same with lead on the ground post of the battery and check your hot wires right at the computer.
Good bright light = everything ok.
Weak-dim light = bad wire or connection
This will make sure the hot and ground circuits are good.