Mercedes ( and some other cars ) put a bunch of ground eyelets on one stud. When the bottom one ( closest to the chassis ) corrodes it takes the others with it. Problem here is the upper grounds will seek ground through each other leading to a while raft of odd behaviors.

When the high beams are on more power is consumed and a different ground wire is used ( if there are two bulbs in the system ). Think of how a dirty battery terminal works at low power but fails at high.