Big thanks to SLK World and DALLASSLK for the detailed post on how to fix! This can work for all models with similar shifters for certain years.

SHIFTER STUCK in N and D, TRANSMISSION not shifting - MERCEDES FIX Video R171 and similar models

SLK350 Shifter Stuck in N and D, cannot Reverse or put in Park Fix video - 2005 Mercedes R171

SHIFTER STUCK in N and D, cannot Reverse or put in Park - MERCEDES FIX Video 2005 R171 and similar