Ask someone to see if the brake lights will come on while you step on the pedal. If no lights, check the brake light switch and the fuse for the brake lights. This is pretty much all you can do without the OE SDS diagnostic. I'd suggest to find a good MB specialty shop nearby.
Just my luck I made this $1200+ repair last month @ MB of LB -- parts required were the Shifter, Pin, and Lock. Removel and replacement are made inside the car. It did not look hard for a DIY, I was going to do it myself; however, I was not sure the shifter replacement was the answer. And I would not have not been prepared/aware of the lock and the pin replace. Hope you can diy and save - good luck