It might be a little late for a warranty repair . . .
If I recall correctly, dealership numbers were either assigned on the build sheet for the car at the factory and then possibly used at the Mercedes Prep Centers in the US for filling out the window sticker.
Your best hope of finding the dealer ID is to have your local dealership check the VMI history on their Mercedes network system, which should identify the original dealership that took delivery. There is a caveat to that, some cars were "diverted" if there was a customer that wanted the car, but lived in another sales area. From personal experience, a Mercedes we had was originally destined for a dealer in MA, but some strings were pulled and it was sold in NJ. It even went so far as swapping out a drivers seat before the sale, so the original build tag isn't accurate.