Each interior light has a constantly-connected brown ground wire.
Each interior light has a switched brown/white ground wire.
When the light body or switch is turned to the "OFF" position, there is power on the bulb, but both grounds are disconnected, so the light will not glow.
When the light body or switch is turned to the "ON" position, that light is connected to the constantly-on brown ground wire, and that light will glow.
When the light body or switch is turned to the "AUTO" position, the light is connected to the switched brown/white ground wire. If the brown/white wire is grounded thru a door or hatch switch, or by the time delay relay, that light will glow.
If the brown/white wire is grounded by one of the switches or by a short-circuit to ground somewhere, the interior lights that are in the "AUTO" position will glow all the time.
A more insidious problem occurs when the brow/white wire is grounded and all of the interior lights are in the "OFF" position - no visible light will glow, but the red lights in the edge of the doors will be always on, draining the battery.
The interior light time-delay relay only holds the brown/white wire grounded for a short period after it has been grounded by a door or hatch switch. (Later cars only.)
Wally Plumley
928 Specialists
