I have a 2004 ford f150 4.6 2wd and the tail lights, brake lights and turn signals aren’t working. The previous owner replaced the bed with a 2005 4x4 bed, and said when they did they cut the wiring harness. Until earlier today the tail lights worked but when they’re on the brake lights did not, or when the tail lights were off the brake lights worked. But then out of nowhere I lost the tail lights and brake lights. I thought I could maybe get underneath and find something that came undone or something simple to at least get the tail lights back but I can’t figure out what’s going on! If someone could help with any advice or where to start or maybe if they have a wiring diagram that would be amazing! Thanks again!

 

Sounds like you've lost the ground for the rear lighting circuits. On this series of trucks, the ground for the rear lamps, instead of attaching at the rear of the truck, is fed back into the cabin and is attached to the inside of the body behind the left hand kick panel.

You'll need to start by inspecting the ground connections, then, as necessary, checking the harness and its connectors along the left frame rail to the back of the truck looking for corrosion or other faults.