there should be aonly one wire is energized in the start position, it goes to the coil side of the rignition resitor to provide 12 volts to the coil when the starter is engaged, it is either brown or blue, CRS disease I have a 1969 FSM, not a 1968, that i can look that underhodd schematic up and tell you what color it is in 1969 I'm pretty sure there the same colors form 1962 and later for the B bodys
BTW, are you looking for a spare ircuit to use for some other purpose? If so I may not be able to help on that, sorry