66 c10 tail light wiring problems

[patinac10]

i am having trouble with my lights not working. The running lights work but the passenger side brake light does not light up while brake is being hit. has anyone had this problem or have a wiring diagram to help? wires are a mess so it is very hard to tell if anything is not right. not sure if its had a 65 bed put on it or if its the 66 bed but has no back up lights, thanks for any help
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