2001 3.8: Turn on headlights, cruise quits working, radio quits intermittently, left headlight goes dim, left front turn signal goes very dim. Turn off headlights - everything works fine. Ok fellow fanatics, give me your best guesses!

 

How old is the battery? Do you have UDP? Has the alt been tested?

 

Check a wiring diagram for headlight grounds. You have a bad headlight ground that is trying to find a ground elsewhere. Could be in headlamp socket or a broken or corroded chassis ground wire.
Good Luck,
Corky

 

Ground was loose. Easiest fix ever. Whew. Panic Subsiding. Thanks!

 

Glad it was a cheap fix for you. Bad grounds can do funky things to components. I find a lot of ground problems after someone does an engine or trans swap. Few people pay attention to grounds - the ground wire is just as important as the power wire.
Thanks for posting feedback,
Corky

 

One of the things I learned as a kid was that the chassis ground is mostly for moral support. Run a ground wire AT LEAST as big as the hot lead.

I used to have a hum come from the speakers when I plugged an mp3 player into the cigarette lighter (for power) and the AUX jack on the stereo (in a different car, not the Mustang). Running a separate ground solved that. And I have a big, fat ground wire run from the battery to the radios in the trunk on the Mustang.

 

This was a really good lesson. I'm a novice in car electrical so studying a ground chart was a new experience. When I found the headlight ground I just had to laugh. It was right in front of me under the hood by the headlight- finger loose. Two quick turns with the ratchet and all was well.