Lights all sorts of messed up.

So, there are many things wrong with my exterior lights. My running lights don't work, reverse lights don't work, my brake lights dont work when my headlights are on, when I have a turn signal on and I press the brake, the rear turn signal will be opposite of that which my front is, or should be, and my hazards flash so weak you can't see them. The previous owners added a new bumper with a hitch and trailer lights, and they would've had to change some wiring around. Any ideas as to how to fix it?