W124 Sunroof tilts, won't slide, only lowers one side
 

I just bought my first Mercedes last weekend. Originally a VW guy (mk4 GTI and Passat W8), I've had my eyes on one of these for years. It's a 1994 E420 with 215k miles. It's got some minor issues, but really runs pretty well. Luckily I'm pretty good with a wrench so most problems I can fix myself as long as I can figure them out.

Today's issue:
The sunroof will tilt up and close fine, but when trying to slide it back, it only drops down on the passenger side, then seems to get jammed and stops there. The motor seems like it's irking fine, and when trying to manually open it, the same things occurs.

I've got no clue where to start. I've tried "helping" the roof drop down by pushing on it from the outside, but it doesn't seem to want to go and I don't want to break it. Obviously, it seems like if the drivers side would drop down, then it might open correctly, but I just don't have an idea why it won't open. Suggestions where to start?