I have a 1980 300TD and recently the interior light doesn't come on when I open the drivers side door (never came on with passenger). The light works fine if I manually turn the switch on. The rear cargo area comes on when I open the rear doors. Where should I start to trouble shoot this issue?

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Current fleet-
2015 BMW 750i xDrive
2009 BMW 535i xDrive
2004 Chevy Avalanche 4x4
1993 Mercedes 300SD
1993 Mercedes 300SE (parts car)

Previous Benz's-
2001 Mercedes SL500
1983 Mercedes 300SD
1980 Mercedes 300TD
1996 Mercedes E300D
1999 E300DT
1984 300TD-T
1988 Mercedes 560SEL (parts car)
1979 300SD
2003 E500
2004 S430 4Matic
2001 E55
2003 E430