My left front headlight doesnt light up even though the lamp is fine. It used to light up if I revved the car up and it stayed on. Then it just went out and doesnt light up - once in a while it will come on at freeway speeds. Also, my brights have been out until yesterday - they started working - seems that the faster I go, the more likely they work.

Any ideas?

 

Bad ground. Check all the wires and connections going to your headlights..........especially on the side that isn't working. If there isn't a good ground, they won't work properly.

 

If not the grounds, check the contacts for corrosion. Take a small thin screwdriver and depress the tab that holds each female connector in the connector housing and it will pop right out (do one at a time). Mine were black from corrosion. I polished with 600 sandpaper and WD40, good as new.

Bend the tabs up a little so they will catch and hold when you reinstall.

 

I'll second this from personal experience. EXACTLY the way mentioned, except I used some di-electric grease afterwards for a longer lasting fix

 

Cleaning off the terminals fixed the headlamp that was acting up - also checked the grounds. Seems as though the ground wire (green) connects to both headlamps and goes into the wiring inside the firewall. I dont see where it is grounded outside the vehicle. Could this be the reason the brights still dont work? If I need to splice in a ground to the frame - where do I do it?

 

The green should be the power side, ground is usually black or brown. I would lean towards the dimmer switch if either the high or low side is not working.

Disconnect the plug on the dimmer switch, use a test light to determine which side is power in, then jumper across on the plug to both light circuits. If the lights work, the problem is the switch, if they still don't, then you have a wiring harness problem.

 

I coulda sworn that the ground on mine was on the firewall ?

I have a 73, so that my differ from yours.

 

On most of the cars there should be a short ground from the back of each headlight to the core support.

Another thing to check is that you have a good chassis & frame grounds. Since the core support and the subframe are on rubber bushings, there should be:

a) ground cable from battery to alternator bracket
b) small ground from battery to fender
c) 2 ground straps from the back of the motor to the firewall

Most often if the motor was out, the grounds weren't connected back to firewall, and will lead to electrical gremlins. I usually run another ground from the bottom of the firewall to the subframe also.