Trailer lights not working when brakes applied

limited info generic answer 4-6-7 pin w/wo brake --brake controller etc if you were fixing lights while hooked up possible to have shorted out some circuits or hace wires crossed or short check your fuses and abs can be a shorted sensor not your brakes themselves -----hook up a scanner and check for trouble codes

Look for blown fuse in the fuse panel. Make sure boat is not connected, replace fuse, see if lights return. If they return, you have a short in your trailer lights or connector (car or trailer).

The wiring on the trailer plug or the van is incorrect. Make sure both are wired the same.

is trailer lite fixture grounded good try and wiggle fixture when u hit signal. next try wiggle pig tail on trailer , still no signal. it must be wire from truck, trace back to connection and check all splices. to see if signal light works on trailer unhook pigtail and get a battery charger or another battery, connect a jumper from positive side to signal spot on trailer hook up. hook up ground as well see if light works if not its in the truck for sure.

If happens only when the boat trailer you might check to make sure you have a good ground. Might also try removing trailer bulbs one at a time. They can cause strage things to happen if they're bad. (don't have to be blown)

Check the bulbs in the trailer. If you know someone with a trailer that you could hook up to try that too. A large majority of trailer lighting systems use a simple circuit pattern. Like Christmas lights. . . one goes out they all go out. etc. Some boat trailer lights will burn out if they are hooked up when it's backed into the water. The pain of trailer stuff is figuring out if its really the trailer or not. When the cover is off the lights make sure you take a good look at the guts of the light. The brass pathways will actually arc and melt. Ruining the light. If its not the trailer start checking the vehicle connections. Remember not to overthink things and start with simple stuff first. Oh and recruit a helper to push the brakes and run back and forth it will speed things up greatly.

the wire you maybe talking about could posibly be for the trailer brake controller, which you have to buy serperate i'm not to sure how much it will run you but i would look into that because that is what the extra wire is usually forwith the 7 prong trailer plugs

I had the same problem with my 2002 Silverado last summer when I first bought my boat and was getting the trailer ready to go. Like an idiot, I rewired the boat trailer on that side before I decided to look at the owner's manual. The pickup has separate fuses for right and left side of trailer. They're located in the big box in the engine compartment. I replaced the 10 or 15 size fuse and haven't had a problem since. Hope this helps!

Your turnsignal flasher needs to be replaced with a heavy duty flasher.
also, your boat trailor wiring harness is grounded incorrectly or the harness pig tail is wired incorrectly.
this is why when you excite your tail lights and brake lights that your reverse lights come on and the reason that the lights are dimmimg is because your circuits are looking for the correct ground.