You probably have the 70 amp alternator which in reality barely cranks out 55 amps at 3000 rpm. At idle it probably puts out 15-20 amps which is not enough to keep up with the latent demand, much less aftermarket headlights or the auxiliary fan. This is a known problem with this alternator. You need to upgrade to at least the 80amp alternator that was installed on later model 124's or if you crave more juice (aftermarket stereos and amps) upgrade to the 143 or 150 amp alternator installed on later MB's. Plenty of posts on performing this upgrade. Mark