Electrical issue with my 1990 300D

Definitely a short or failed connection. First, check the 80 Amp strip fuse located in a black box on the firewall, left (driver’s) side. ANYthing less than perfect, REPLACE it. If that’s not the problem, CLOSELY inspect ALL connections between the ignition switch, battery, alternator, voltage regulator, and fusebox. Also, make certain you have updated all other fuses, to prevent future electrical issues.

You must first understand how the controller works, initially its a timer circuit, do you allow it to fully run before trying to start the engine, not only the dash light to go out but the relay to completely cycle off by itself before trying to start the engine. Also pay attention until you replace it or find an additional problem, are you hitting a good size bump in the road for the light to come back on? Bouncing the relay box around may jolt the contacts together where they may stick together, under normal conditions, there should be no power for the short circuit to happen anyways. If you try to start again after the initial timed circuit has completed and the unit has a fault, it will come back on when it should not, this is where you inspect for ignition switch problems while driving down the road, do not hang a lot of keys from the ignition switch, the extra weight is a switch killer over time.

You may elect to upgrade to a manual system available online.