I have 99" xj Cherokee and an issue just started happening when I press my brake all my marker lights turn on - I do have trailer wiring but it's not grounded anywhere- everything turns on including all dash lights,turn signal lights and brake lights wont turn off when the light stick is pulled out.

 

The 1st thing is check the light bulbs to see if the filament is shorted or just remove a bulb one at a time and see if the problem goes away , if that's ok then look at the wiring and maybe trailer wiring if you have that .

 

Do the brake lights come on with the parking/position/whatever they're called now lights?

I had that very problem, but only with LED's (after about 4 years) but problem disappeared when a spell of particularly wet weather passed.

 

When this kind of issue appears, assuming no one rearranged your wires, it is most often a grounding issue. This happens when the grounds are bundled together, the marker light ground, the brake light ground, and someplace the ground for the the brake light to the main ground (ie body or lamp house) is high resistance.
What happens is the brake light electrons cant all pile thru its ground wire easily to the main ground point, and find an easier path through the marker light filiment.


The first thing to check is the grounds at or near the brake lights, likely 90% of time this is issue.......

Check BOTH tail light assemblies, make sure all the bulbs are seated well, all wire connectors are clean and tight. Check the tail lamp ground body body point, which is hidden behind the left side rear quarter interior trim panel, inside the quarter panel near the fuel filler. inspect, remove rust, clean any terminals that are not shiny, sand the body any place terminals touch body, clean the wire side of the securing nut, and any washers, you want clean shiny metal! then reassemble, nice and snug, a lock washer should be used. I altered mine from a sheet metal screw to a nut and bolt that can be tightened much better than the stock sheet metal screw, ( this is optional, )

Lettuce know how it goes!


PS this is not only a jeep problem, just about all cars are suseptable to weak grounds causing other things to turn on. this even happens to 60 year old cars, and modern cars. Its a car thing!

 

Turns out my main rear wiring cluster was extremely wet. It was raining all day and started when I started to drive the rain the problem arose, it persisted for today but when I got off of work it had all tied out and it works just fine now, and thanks for help.

 

Interesting, so like mine - except there was no evidence of damp in the light clusters (& I was investigating the problem in the rain, under the shelter of the liftgate) it's just that the weather had been particularly wet, for a couple of weeks.