Check the wiring, you have a ground problem at the right rear taillight, and are getting 12V on the stop lamp wire when the headlights are on (this is called feedback, by the way, something automotive systems are VERY prone to!).
I'd bet the ground connection on that side is a bit corroded or the wire is broken off.
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!