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.

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.

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.

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.