It's either the cable(s) or the crappy, flimsy mechanism that tilts/slides the glass. Cables are about £25 per side and the operating slides around £120 each complete with cables attached...Genuine LR

(For what it's worth !!).
Replaced mine last year-took about an hour. Don't have to remove the headlining, just the aperture finisher trim and drop the interior light.
TIP.
With the glass and slide/tilt mechanisms removed, check the drain tubes for blockages. I used a length of spring net curtain wire, taped up on the end to prevent any sharp edges from piercing the wall of the drain tube. Also give the mechanisms and cables an extra greasing.