Wiring diagrams are available from all the vendors. $16 maybe?? Great investment. There's a DVD version for $25 that is colorized, even better. If I wasn't at work, I could look it up, but everyone should have their own.
On a 1966, the power to the switch is a Black/Orange wire circuit 25 from the fuse panel. If you have power at the switch on that circuit (you have to confirm wire color for 68 but likely the same) the only other common point appears to be the switch.