I realize that this is going to sound pretty dumb BUT....... Take the turn signal apart, totally, and wire brush all the connections very well. It sounds like you are getting a feed back through the ground. Do the same with the headlights. Put a little dielectric grease on the bulbs before putting them back into the sockets.
First thing is to check for a burnt out bulb in either the front or rear turn signal for the side that stays lit. Second, if the bulbs are fine then check for a bad turn signal fuse/cannister. Hope this helps.
If you turn on your 1999 mustang v6 car and the right signal light stays on and doesn't blink and you switch the car off and then the left signal light stays on without blinking, then you need to see a mechanic.
If a signal light stays on, but does not flash on a 1989 Pontiac, there could be a light burned out in the turn signals on the exterior of the vehicle. It is also possible that the flasher unit has gone bad. The flasher unit is under the dash on the driver's side.
It means that a light bulb is out on the side where the light solid (not blinking) it could be in the front or rear of the vehicle.
You need a turn signal switch, but spray WD 40 up through handle first & work back & forth, sometime's that cleans the contacts !
No, series circuits are not used in car headlights, because if one headlight were to go out the other would also, because the one light would make an incomplete circuit. Headlights are wired as parallel circuits because when one headlight goes out, the other is unaffected and stays as bright as it was before the other light went out.
Recommend you get a stock signal light switch. This may be your problem.
This may be a bad dual filiment bulb (1157,3057) dependin on year. Also the bulb may be in backwards. This is hard to do but is a possibility. If the signal indicator is on and stays on it is most likely a bad bulb. Otherwise you have a broken wire or a short in the lighting harness.
You know a turn signal has burnt out when the light on the dash flashes very slowly, or just stays on until you cancel the signal. I don't think there is any way to know that it is about to burn out.
In some vehicles, a consistently lit turn signal indicator dash light acts as an error message. What the light could be saying is that either the bulb is burnt out and needs replacing, or there is an error with the related fuse.
Sounds like you have a short in your wiring harness for your rear lights. As for your dash turn signal light, your dummy light is just acting up. You probably need a new wiring harness for your rear lights.
Sounds like you simply have A burned out bulb, Turn the signal on and look at all the lights to see if any are burned out. Could be A marker light if its in the turn signal circuit.