I just did my right side on my 190E.
The power mirror is the same for the 300E.
BE CAREFUL putting in the new glass. Very hard to put in! I just pried out the inside cover, took out the 3 cross head screws undid the electric cord (also screwed in centre) and replaced the glass in my downstairs work bench.
I had the best results putting the mirror in a vice, (towels around the outside) and aiming the frame for the power mirror motor directly upward then progressively and slowly pushing the new glass down. Then it'll firmly snap into place....
Good luck with your car brother!
190E 2.6 - 244 000 km