My Dodge pickup is doing weird things. The heater fan shuts off if you turn on the parking or headlights. The cruise control shuts
off if you turn on the parking light or headlights. I first noticed it with the cruise then a week or so later the heater fan. Any ideas , thinking
about changing out the headlight switch....

Frank

Does it have a multifunction switch on the top part of the steering column, below the steering wheel? That one switch controls many of those functions, if the truck has it.

if not, I would start looking for a corroded or broken ground wire (s).

I'm thinking possibly a grounding issue since the positive side of those things aren't directly connected, but that's just a guess on my part.

Its not a headlamp switch problem. Headlamps are on there own and do not rely on the key being on. I would look at ground issues.

X3 on ground. Not sure if that generation uses a wire from the core support to the battery like the older trucks but that was a common failure.

Never mind/delete.

My son looked things over in the engine compartment and didn't find anything so far , but we are still looking.

One thing my wife reminded me of , when we hook either of my trailers up ( I have a flat bed car trailer and
my 12 foot swap meet trailer ) the trailer lights come on and stay on , even when the headlight switch is shut off.
That is as long as the key is turned on.

Also I have a drivers side front turn signal that is lit up on the dash , like it out , not working and flashes fast when
you turn it on to use it. When a trailer is plugged in , it works fine. Turn signal lite in the dash is off and it flashes like it
supposed to flash.

Frank

It definitely sound like a wiring problem.
There's a main connection to all of the wiring that goes to the rear of the truck behind the LF wheel well fender liner at the bottom rear right next to the cab mount.

The fuel pump wiring is also in that wiring bundle, so if you disconnect it the truck won't start (along with all the rear lighting, trailer included won't work).

If the trailer light issue is some that started with the trailer all along, I would also check to verify that the trailer plug is wired correctly, since there's a positive power lead wire in the plug on the truck.

Bad ground? You've checked ALL the fuses, right? Run a separate ground wire from the trailer to the truck, just for testing and see what happens. 2¢

Check the 7 pin trailer connector on the truck have replaced a few of them for internal corrosion problems,causing shorts&other problems.

Do you have a heavy duty flasher installed? Any led lights? The 7 pin connector is it factory(trailer package) or did somebody install it? Do you have a brake controller installed? You might want to check the wiring of the plug on both the truck/trailer.

thanks for posting those trailer plug wiring diagrams.
i didn't have those, so i saved them for future reference.

It does but that switch only controls the directionals , wipers , and high beam. Cruise is on the steering wheel , but those all go thru the clock spring down on the column ... radio controls in the clock spring also if it has the top line radio.

Heat is it's on separate thing .