Dave,
If I recall, in '68s the gray wire is for the light in the column shifter. If you have it it would illuminate when the dash lights would turn on. If you apply + 12 battery to that wire you would be applying power backwards to the dash from there in place of from it's correct source. That makes me think it is in the + supply instead of a bad ground. If the dash grounds were bad, +12 power applied still would not complete the circuit and illuminate.
Jim