98 mustang gt help

Test for key on power in and out of I/P fuse #18. Use a test light that will "load" the circuit.

1996-1998 BJB & I/P Fuse panel layout

Test the LG/R wire with a VOM meter. How much power with the key on. Repeat the test with a test light.

Now disconnect the small alternator connector and test the unloaded LG/R wire with the key on.

Note, I have helped more than one person with an alternator charging problem that turned out to be a bad ballast resister inside the cluster.

Howto perform charging system voltage drop test

Cooling fan or inside fan?

It would help if gave the fuse number using the diagrams provided.

If looking for an educated guess, I would want to know the condition of the Battery Junction Box (BJB). I'm thinking water or rodent activity inside the BJB.