Due to the age of the car and that all of your problems can be deduced to your doors I would start by measuring the contacts in the doors (the ones that among others trigger the light to go on.) If you find any contact problems in them you should start by spraying them with some kind of anti-oxide-spray/fluid. The contacts can easy be affected by oxide because of their vulnerable position.
Emil Ísterman, Stockholm - Sweden
MB SL500 -93 - Silver
MB 230CE -84 - Silver
MB 230E -82 - Yellow
MB 180B -59 - Green