You replace the plug and wire for a while, and run ok, so, I don't really think you plug the wrong wire to the wrong plug.

ok, you press on the brake and reverse? isn't it? well, I think you can check your wire, whether it have a good conduction to the plug and the distrubutor, it possiblly causing by the bad connection, so the spark is not strong enough in high tourque condition, and may be you can try hard acceration, then you might feel it miss fire, this is also the same thing.

I have this situation before, but not in the Mercedes.

hope this can help.