Mercedes A/T mechanics?

Well, Mercedes once owned Chrysler so Ill ask (yeah, it was a stretch) 91 560SEC. B2(?) piston in A/T is bad, its stuck in reverse or something, wont go into drive. Was trouble for a while and finally stopped altogether. Its my sisters car but she will almost give it to me as she already bought an A6. Her mechanic wont touch it as he's not a Benz guy but he told her it is actually accessible from under the car, no tranny removal is necessary. Its just a plate that covers this annular (?) piston. She wanted me to look at it for her and didnt know what to expect. Is it doable on my back or is it a rack job? Beautiful Black on Black with a pimp 24K grill that was found in the trunk still in platers plastic bag. Imagine your first car at 17 being a MB 560SEC or a Lexus SC400?