My 300DT does the same thing, but the 300TE doesn't. I suspect the lamp or the seatbelt warning lamp is bad, or the ground is dirty. The dome light is dimmer on the 300DT, too.

I put a new dome lamp in the 300TE last year, as it was burned out when I got it.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!