An interior lamp with only 4 wires is one of the older types.
These lights do not dim gradually before they are turned off completely but will shut off at once after a certain delay.
The pins mentioned must be TK, 30, 31 & 15 (instead of the second mentioned 30).
TK stands for "türkontakt" which is German for doorcontact; this is the cable which is connected to ground when the door is opened.
30 is connected to permanent 12V
31 is connected to ground directly
15 is connected to +12V after the ignition key is turned into position 1 or 2.

Switches of these light units tend to break down occasionally and some of the electrical contents in these lights (resistance diodes)burned out to often (at least in the units used in the Netherlands with the flashing seat-belt insertion).

greetingz & good luck

__________________
1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)