Reverse light wiring problem

Ok , I have a reverse switch on the trans that works , wires from the switch are Blk/Blk trace running up to engine bay , have a T shaped plug .
Under the hood the only plug that matches on the harness I plugged into lights up the EGR lamp on the dash when the car is put in reverse , kinda tells me this blk /light green pig tail isn't where I need to go .
In the trunk the back up lamps have violet/ white wires and go to the harness on the drivers side in the trunk but I can't find them under the dash or going through the firewall.
The car has stock wiring from 1975.
Any of you guys tell me if the blk / blk trace wires coming up from the reverse switch are plugged in under the hood or do they route somewhere else ?
1975 Duster , 360 4 spd.