Has anyone had this problem, it only seems to happen when the car is hot during normal driving. I have changed the switch 3 times and it didn't help. It happens during the cruise nights when i've been driving awhile. The car does not overheat but when it gets to the top temp range the lights will start going on and off on thier own ????

A bad high / low beam switch will cause that.

If you have halogen head light bulbs or other non-stock brighter higher wattage bulbs, it can cause the internal circuit breaker, in the switch, to trip off and on.
Usually the aftermarket switches have an updated breaker that will not trip as easily.

A bad connector plug, anywhere in the lighting circuit can get hot and cause trouble also.

I would check all the plug pins and connectors. Look for corroded, signs of hot or burned pins or plastic. Look at both sides of the plugs on the bulbs, dimmer switch, big plug on the switch, etc. Screws attaching the dimmer switch to floor. (big deal on some GMs)

Also check the ground wires that bolt to the radiator support behind the headlights. Scrape off paint from under them.

Make sure you have a body ground. The battery grounding cable should attach midway to the body at the voltage regulator screw and then to the engine block. If it is not, you can run a smaller 10-12 gauge ground wire to the body from the motor or battery terminal.

Well Ive replaced everything, put on an extra ground but after the lights have been on awhile they flash on and off. just the headlights nothing else. All the other lights stay on and function properly. Is there anything else I could check? I do have newer halogen bulbs that I havent messed with yet.Very frustrated, Danny

You're probably going to have to install a relay to power your new bulbs. The headlight switch will then just be powering the coil of the relay, and the relays contacts will control the headlights. Power for the lamps would all need to be wired into the dimmer switch, with power taken from the main battery feed, which is the thickest yellow wire in the cars harnesses.

Anyway....The original 73 Mustang headlight switch had a higher rating, than the 71-72. See if you can locate a 73 only switch, it might help?

Get lower wattage bulbs, or well know brands made for direct replacement. Some of the cheap blubs are 75-100 watts, which is too much. You can get more light with a GOOD brand, lower wattage. I use Salvainia
Silver Star's in my truck...no problems.

Maybe you bought sealed -beams...what brand???

Mustangsunlimited.com lists the 71-72 switch #sw1245 for $50.

The 73 switch is #sw1226 for $28.

According to the shop manuals and owners manuals, the original 73 switch was rated a few amps higher than the prior switch. Either switch number is called for 71,72, 73, but there is a difference between the two, and it's the switches circuit breaker rating.

Rockauto.com might have two diffent switches too?

What i'd do if NOTHING worked.....remove speedo head, take left side plastic dash vent assembly, cut about 1" hole in it's side, install 1' dia short hose there, and have it run down to the H/L switch to help cool it?

Try unhooking one headlight bulb. See if it still does it. If not, you will know the internal circuit breaker is overloaded or defective.

A stock switch will trip with halogen bulbs, mine did. I got a replacement switch from an auto parts store and it fixed it. That switch fits many cars and trucks for a lot of years, even AMC cars.

Sorry it took so long to answer. The problem is fixedfor now. I unhooked one bulb and the other one stayed on and never blinked on and off. as soon as I plugged the second one back in the started blinking on and off again. I went to autozone and they had 2 part numbers for a light switch. a number for 71 AND 72 models wich I had and a different number for the 73 model year. I purchased the 73 switch and the problem is gone.thanks again to everyone who helped, Danny

I understand the problem's fixed. I have the same issue only the lights flicker slightly and the pulse rate increases as the RPM's increase. I read here sometime ago regarding changing out the dimer switch and that failed. I wonder if I have the wrong switch. Were your lights flickering or turning OFF and ON?

Sounds like AMK's Voltage Regulator!!! flicker,flicker,flicker,,,,
Just my .02
Chris AZ

and changed the regulator and nothing new...I thought perhaps the alternator needed to be gone through. Perhaps not getting full contact on the armature? Could explain the increased flicker with invrease of RPM's?

Ron,
Did you replace it with the same kind? Or a electronic one?? I bought a electronic one, and put the big org cover on it and it works fine. Thy that..
Chris AZ

MINE WERE ACTUALLY TURNING ON AND OFF, JUST THE HEADLIGHTS NONE OF THE OTHER LIGHTS WERE AFFECTED.RPM CHANGE MADE NO DIFFERENCE.

ALL my lights flicker, and I mean ALL. Nice and bright, no voltage drop, at least as much as I can tell with the orig gauge's...LOL...