01 tundra brake light problems. HELP!!!

hello, i recently bought an 01 tundra SR5 access cab a few months ago and i realized i had a big problem but not until after i purchased the truck. my running lights work BUT my reverse lights, rear blinkers on both driver and passenger side, and brake lights don't work. i started with the easiest check and looked at the bulbs. all the bulbs were fine. checked the fuses and all fuses were great. i bought new tail lights with all new bulbs and a new wiring harness that came with it. this wiring harness had the boot connecter already ready to go. these are simple plug and play lights. when i went under to check the connecters i saw the previous owner had wired in a trailer light system that is connected to my battery. i checked the wires for the trailer lighting kit and the wire was slightly burned. i cut backa few inches and we crimped the wires. when i cut the wires i noticed a blue corrosive color in the wires. i checked the fuses tied into the trailer kit and both the 10A, 5A, and 20A were all popped. i went and replaced all the fuses and still nada. so i disconnected the trailer wiring harness from the plug and play in the engine bay near the battery and the running lights still work even with the trailer kit unplugged BUT still no lights. i have 0 idea wgats going on and if someone could help i would greatly appreciate it.

 

Have you looked at the turn signal flasher/relay. Just a thought! Also, if you know how, check the other associated relays.

Were it me, I'd pull the entire jury-rigged trailer harness too.