PEH
I don't know about the 126. It's possible on the 123 but not the 124 so even though there might be more space behind the dash of the 126 it's largely related to the design. On the 123 you have to remove the glovebox and center vents. I found with my smallish hands I could reach the actuator with my right hand and use my left hand to manipulate tools through the center vents. The main problem is that 80% of the work is done by feel (unless your x-ray vision is working). On the 123 the job wasn't hard or time consuming as much as it was just fussy and frustrating. Depending upon what you need to do you might have just enough room in your 126 if you get a bit creative. As you probably know, a common failure point is the rubber bladders in the actuators that crack and leak. If all the vacuum plumbing looks good and nothing is obviously broken you probably need a new actuator (the early ones are rebuildable but the later ones generally are not). I have to admit it's great having those center vents working on a hot summer day. Sounds like it's time to break out the mityVac. Good luck.
LRG

__________________
LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold