The worst...W203
 

Quickly, I'm working on a 2003 C320. Has a battery drain (about two days to kill it). I get no interior welcome lights (from the unlock signal, or the door switch). When the individual interior light buttons are pressed, they illuminate dimly for the brief moment the switch is pressed, but never come on. Instrument cluster warns "inop light sensor", but the auto headlights function fine. If anyone has info or this circuit diagram I'd truly appreciate it.

Common draw issues are from interior network NOT going to sleep.

Most common DRAW issues are the seat modules, then the rear SAM, & then the dome lamp assembly.

Sounds like you have a dome lamp issue. Diagnosis is done with scanner OR the old fashion way of connecting ammeter & disconnecting the component in question.

The dome lamp is called the OHCP (overhead control panel)

you seem to be a life saver. I wont be at the car until tommorrow, but it sounds like the place to start. Do you know the part number for the ohcp? i need to price it, im sure its too much...

Yup its a common problem with W203's, the seat modules are pricey even at wholesale. Browse around ebay, i found an updated set on ebay for $100. I cant remember what the part number change was, but ill search for the thread on it in the mbworld forum.