not an a-body but need help please

bought a 70 d100 stepside this week and all the lights work but the pass brake light. cant find where it feeds to the wiring harness. tried wiring from the driver side to the pass side but if i try to use the pass signal with the brake light on the blinker quits. any help greatly appreciated. tried some truck sites but cant find a place comparible to the folks here. thanks:violent2: