If you've got the orange + up from fuse box to brk lamp sw already, then you should hook up an 18 ga white wire to go to the white wire on the (67 style column) turn signal wire connector (dark green wire should be next to it) on one end.
Mine is a newer van tilt (early 70's/no key on column). It has the wire as black w/white tracer, but still on the end next to the dark green one.
Just found wiring diagram. Google 67-72 chevytrucks.com, then go to 1970 chevy wire diagram. Good luck, Sammons