Interior courtesy lights quattro Digest, Vol 109, Issue 22

[Scott]

Something is taking all the power from your battery.

Check the battery itself.
older than 6 years it could be weak with no reserve.

The older C chasis (5000) had a spliced power line to the engine compartment, which when corroded would heat up and not transfer all the power available.
Heated connections that expand may not pass enough juice until cooled off and "shrunk" back together............

Something is sapping your juice.
Friction 
Starter
Bad Alt not recharging battery fully.

I suspect the battery or some circuit drawing amperage.

You could meter (VM) the fuse box and see if there is a current draw across a fuse, etc.,

Investigate and tell us what you get

2. Interior courtesy lights (Jay M)