Helpful or not, here's 2 cents.

I have the un-powered converter in my '93 300D and it doesn't turn on the lamp out bulb. I don't believe I've had a trailer light out just yet to see if that triggers the fault.

My friend has the same year and model and we installed a powered converter in his. Mostly because his bulb out light was triggering at the time. Turned out to be poorly spliced wires for a headlight. Oh the gifts you find on a used car.

A hitch for a W126 you say? My brother needs one of those. It's a '91 350SD with a 3.0L already swapped in.