I just ran into this issue on our 1987 Cabriolet, when i turn on the headlights my back lights stop working as well side marker lights. if i move the light switch around i can get back lights & side marker lights trunk lights and interior dash ligths to work except for the headlights. this just started happening yesterday after the car lights where functioning correctly.
1 - Checked the Fuses and they are good
2 - when headlights are switched on turn signals lights work as well, all other are off (Side marker lights, back lights, trunk light, dash lights)
3 - when Light switch is moved around the Side marker lights, back lights, trunk light, dash lights work and the Headlights i cannot get to turn on.
any ideas what could be causing this?