the wire you have hanging is the hot wire, and the only one to the light - ground for the light is the light mounting to the cab
if you can see under there
the terminals have code letters that indicate what they are - there's no brown/white runner shown in the wiring diag
the interior lite is controlled by the rotation of the knob - one way dims dash lites, other way [click past full brightness of dash lites] gives the interior lite - find the terminal that has the dash lites on it and the other will be right close - may be it was disconnected cuz the switch messed up and the PO couldn't turn it off!
Bill
