I'm not sure how applicable this is to your problem, but I recently had similar difficulties on the regulator for the front and rear drivers side windows on my 380SE (1985). The front had a broken/cracked regulator 'gear', and the rear had a broken guide piece.
If your regulator assembly is made of a stamped 1/8" thick steel stock, as mine, just remove the unit (can't help you there, hopefully someone else can instruct for the SL procedure, they seem to vary by year/model), position the broken pieces together, butted as they should be when not broken, and grind out the 2 sides of the crack so they come together in a 'V' shaped trough or valley. Then, MIG weld them back together, filling the valley with weld bead. No need to grind them flat as long as there is clearance when the pieces are moving. You want to retain the thickness of the weld for strength.
As for the guide piece(s), if you are referring to the rectangular white nylon thing that fits into the horizontal channel attached to the glass, do a search and you'll find some posts on replacing it. It's easy once the regulator assembly/motor are out, basically involves drilling out the old rivet and peening in the new piece with a new rivet (attached).