[LH Lead-Foot]

Start a new thread in "Electrical" for "1990 Alt. wiring diagram needed" to see if a forum user has a factory book that show the alternator control circuit for your 1990 C2500(?) Engine size and firewall connector.

I hate to have you do more than you need to find your problem. That is every service tech goal is by visual, wiggle, testing with ground or power, then dis-robe some of the harness to put hands on the "BRN" wire and find the problem.

The lessons learned will last you a life time. I am watching the thread and don't be shy about some photos, but just make sure you have good light. The area behind the brake booster is dark and hard to get into, but this is traditionally where the bulkhead / firewall connector is located.

You can unbolt the 4 nuts at the back of the brake booster and slide it forward leaving master, lines everything connected. Now, you will have to remove the under dash clip on the brake pedals linkage. Rather than a cotter pin, they probably have a clip that is a folded type. The back side is "U" shaped and fits onto the foot pedal. The other half is a long clip that has a tab that clicks into place over the knob end of the brake pedals pin. Use a light that does not get hot under the dash, as the incandescent bulbs burn your face or arms....Don't ask me how I know.

Once upon a time, there where no LED or florescent type lamp for techs, but then house phone where I grew up was wall mounted oak wood.