I took a look at the wiring diagram on the resto site and from what I see that wire is common positive to: Heater light Lighter light bottom pin on direction signal & hazard switch pin 3 and 11 on your instrument cluster
The orign of power appears to be as follows: The red wire on the light switch is your incoming power, this appears (cant tell without a schematic of the switch) to then feed the green wire. The green wire then runs to the fuse box primary side. On the secondary side of this fuse the wire then becomes grey with then runs to the: Heater light Lighter light bottom pin on direction signal & hazard switch pin 3 and 11 on your instrument cluster I would start by metering for voltage at that red wire first, then green, then grey. I dont think that the grey wire up to the shifter makes any difference in the cluster lights comming on because the grey wire is common +. See these wiring diagrams http://1968firebird.groupwhere.org/69wir/69wir1.html http://1968firebird.groupwhere.org/69wir/69wir2.html Hope this helps.
