On my 1980 GMC K15 manual 4-speed, I have the reverse switch out the left side of the trans, near the top/front. There are a pair of wires coming out, one green, one blue. Here they are from ST_352_80_1980_Chevrolet_Light_Truck_Wiring_CK_10_30_Only.pdf:
They are circuit 24 (light green, Back...