Brake Lights Turn On with Headlights

Check wiring harness on the back if the tail light is shorted to the brake light circuit-the wires could be pinched somewhere

I've seen this happen when the wrong bulb is installed. The hint is your comment about it starting right after you replaced some bulbs. Since you already know some bulbs have two filaments, it will make sense when I suggest there might be a single-filament bulb in a two-filament socket. The contact on the base will short the brake and tail light circuits together.

Look in particular at the turn signal bulb you said is a single filament. They are normally a two-filament bulb with a brake light in there too.

Thanks for the replies! I’m really sorry, but I don’t know much about cars so I don't know what to look for to see if there is anything wrong with the wiring harness. I took a pic to see if someone else could tell if it looks bad.

The first pic is of the back of the top (middle) brake light in the back of my car. The bulb is a single filament. Should the bulb be a dual filament one?

I also tried putting dual filament bulbs in the slots that currently have single filaments (the bulbs behind the yellow glass) and that didn’t make any difference. In fact, the dual filament bulbs almost didn’t fit in the plastic holder.

The second pic is of the back of the side lights at the rear of my car. The top bulb (with two wires) is single filament and the bottom bulb (with three wires) is a dual filament one. Should they both be dual filament bulbs?

Rats. I was envisioning the older brass-base bulbs. They have one or two lead button terminals on the bottom. Using a single filament bulb in a dual filament socket will short the two circuits together and make both sets of lights turn on when either circuit is activated.

If your bulbs have the flat plastic bases, using the wrong one should only affect that one bulb, not any others. Since the sockets all go into plastic, there has to be a ground wire in each one. The second wire is the 12 volt feed for the bulb. When a socket has a third wire, that is the feed for the second filament. That's how to tell which kind of bulb to use.

I don't know why I didn't think of it sooner but your description sounds exactly like what happens when there's a broken ground wire. There is typically one in common for many sockets. If this is the problem, you can prove it by measuring the voltages on the wires in the sockets while just the brake or tail lights are turned on. You should find 12 volts on the one feed wire, 0 volts on the ground wire, and if there is a third wire, it should also have 0 volts. If you find 12 volts on one and somewhere around 4 to 6 volts on the other(s), there is a break in the ground wire. Anything other than 0 volts is unacceptable.

We replaced ALL of the bulbs in the back and the blinker lights in the front. No good. I read your post and we tried to measure the volts, but there was a spark and now none of the lights in the back will light up without the headlights being turned on. We are working to figure out what happened and how to fix it. I’ll keep you posted. We must sound like total car idiots…..we sure feel like that at least.

Ok, it was just a blown fuse and we replaced it and are now back at square one. We are nervous about ground testing since we blew a fuse. Do you have any tips on how to do this safely without blowing more fuses?

Electrical is hard because we can't see it flowing in a wire like we can see water flowing.

Sparks are caused by one of two things. Two wires touched and created a short circuit long enough to blow a fuse, or there was a break in a wire and you wiggled or moved something that caused it to overcome that break for a moment. That spark is caused by current starting to flow, which is normal and harmless. A spark due to a short is larger and more surprising.

Check the setting on your meter. If you're using it on the "Amps" or "milliamps" setting, the meter will act like a short circuit. It must be on the DC Volts scale. Most meters have 2, 20, and 200 volt scales. Use the 20 volt scale for the most accuracy.

We fixed it! Here was the problem….it’s so simple that I feel sort of foolish for not finding it right from the beginning….. Inside one of the brake light bulb sockets was a bent pin (one of the metal bits that’s supposed to slide up the side of the bulb). I guess when we changed the bulb 4 years ago we got a little zealous and shoved the bulb in too hard bending the pin and crumpling it at the bottom of the socket in the process. Funny thing is that we replaced the bulb in that socket today and did the same thing without realizing it until later. Long story short, the problem is fixed and my car works great! The brake lights only come on when the brake is pushed. The third (high mount) brake light only comes on when the brake is pushed, not when the headlights come on. And the front running lights don’t come on when I push the brake. YESSS!!!!!

Thanks for all your help! You have no idea how much it meant to us!!

I never would have figured that out over a 'puter so the results proves you're smarter than I am. Now you know what mechanics go through every day. Happy to hear the good news.

I found this !
Both contacts on the bulb it's self had been pushed in from being in the socket so long and the heat from the lights for so many years ... This is why it's best to change both ( brake, blinker, parking, and head light) bulbs on any of them so you know the bulbs are in good working condition...