I have a 77 vette that when the headlights are on the tail and break lights are not. but when the headlights are off the break lights work fine. I think it has to do with a 3 pring wire that is hanging from the fuse box area but I can not figure out where it goes. I am trying to post some pics as well

Thanks

(large pics modified to links)Adams' Apple2007-08-27 10:03:31

Well...either me eyeholes are way worse than folks tell me, or them there pics are purdy blurry!

Can you give us the wire colors on the item in question? That may help narrow it down.

You may have a bad headlight switch, a loose ground for the tail/brake light sockets, or something else is going on here...do the FRONT turn sigs and park lights work with the headlights on?

Here is a better pic. There are three wires comming out of the plug, two white and one black. when the headlights are one both the front and back siginal lights do work
Better PIC



(modified large image to link)Adams' Apple2007-08-27 19:27:03

My guess is bad grounds for the tail lights. I may be some how getting a  ground, though the headlight switch, with the switch in the off position. When you pull the switch on, the intermittent ground is broken and the tail lights will not illuminate. I would try a temporary ground wire from one of the tail lights to the frame and see if that cures the problem. The tail light sockets are notorious for bad grounds.


Scott

The only thing I see in the wiring diagram with two whites and a black is the driver door light switch.
I don't know that the connector you show would have anything to do with the brake/tail light issues...like Scott, I would recommend a separate ground to the bulb socket(s) on one side, just to see if they will work properly then. If so, then I would want to start tracing the black wires out of the sockets back to the ground points, usually around the middle of the rear bumper support, but sometimes also on the driver side of the frame in the rear.
If the turn sigs are working with the headlights on, then the brake lights should also, which brings us to my favorite thing when it comes to brake light malfunctions...the turn signal switch. This would have no effect on the tail lights, tho, sooooo....we're back to grounds...
If you have the tail lights on, and then pull the switch all the way out for headlights, do the tail lights go completely out, or just get really dim? How about the brake lites, same question?Adams' Apple2007-08-27 19:43:50

O.K. first off thanks for all of the help.

Will do, tommorow I will run a temp ground to the socket and see what happens. will update