03 Gt Wiring Issue

Here's some information on how to perform voltage drop tests.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

I know you might not see how over all charging systems tests will help in this situation. However once the concept of how the voltage drop tests is done and how it works it can quickly nail down WHERE the weak electrical connection is. The procedure is to "frame" the test path looking for excessive voltage drop. By framing ever smaller lengths of wire path the exact point of the bad connection can be pin pointed.

For example if you suspect a problem between the battery and battery junction box, place one lead on the battery positive and the other lead on the main terminal of the battery junction box. Turn the key on. Normally the voltage drop should be very small. IF there is a large voltage drop that means the bad connection MUST be between the two leads.