In many vehicles, when the "ignition" switch (even the factory calls it that in the 300D wiring diagram) is turned all the way to "start," many accessory circuits are cut out, to minimize battery drain. The circuit that is blowing fuses may be one of them. Once the engine starts and the key is released, the circuits are re-energized and the fuse blows.

In any case, P E H is right, you have a short somewhere. It must not be a major short or the fuse would blow instantly. The lights dimming and then the fuse blowing suggests that the short is drawing just more than the fuse can support, say about 20 Amps.

I would suggest removing every bulb on that fuse and check out the connections at each socket while removing the bulbs. Start with the easiest and proceed. If you can get a (say) 10 Amp mini circuit breaker at an electronics shop, wire two clip leads to it and clip them across the fuse terminals. That will save you from constantly replacing the fuse as you gradually eliminate one item after another.

Each time you remove something from the circuit (and examine its wiring), try the circuit to see if the breaker or fuse pops. Keep going until you find something, perhaps a wire that is touching something that it shouldn't, a wire with frayed insulation, two crossed wires, a wire on the wrong terminal, etc. It may be a slow procedure but hang in there.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970