When I turn my headlights on my brake lights turn on as well?

Could be a grounding problem with the bulb socket in the back . The running lights don't have a good ground so they go through the brake light , I think . A multi meter is useful to find what has power/ground and what don't .

It's shorting out the positive wire, check under the bed for smashed wires thatvare touching together

As you didn't state the year I'm going to give you all that I know about. Older one had a circuit board that would mess up and cause wrong bulbs to light, bad light bulb sockets they will show burnt ma arks usually inside the plastic and where the ground wire would attach, cross circuited trailer plugs, damaged rear harness due to accident or hitting something on the road. Hope this helps.