rear ended and flasher lights come on in dash wen i hit brakes!?

i was rear ended this morning, i have a tow hitch so the truck had no damage at all but i noticed wen i hit the brakes the flasher lights come on, all the lights work butwen i signal, it will signal but all the other tail lights flash but very dim-ly. spoke to a few people and they said it could have something to do with my trailer wiring harness set up. i disconnected the harness and its still doing it. i need some help

 

no not at all. someone told me the little harness black box might have got jossled and gone bad