Cost to fix hard wiring

It has to do with the braking system. Bought a panel of lights to have in addition to the tail lights. Tried to hook up the panel to the harness of the truck (trailer harness) but it never worked properly. Tried hard wiring it and everything and it still didn't work. Hooked up truck's harness to another truck's trailer to test it and it didn't work properly. Hard wiring of the truck also seems to be affecting the trailer's wiring. Hope this makes sense. Please let me know if there's any other info you need. Thanks so much for the help!