when I bought my car used, I went to the local MB dealer who had performed some service. I believe it depends how you approach them and that you run into an employee who is normally helpful. My MB dealer had a computerized system that could print the last four years of work done on the car and thats what they printed out for me. It is not an invoice, but it lists what was done. Thats how I know my motor-mounds were done in 1999 etc. It might also help, if you give them an oil change to do.
Hope this helps.