You need one of these:

http://ackits.com/index.cfm?fuseaction=Parallel

Its a custom installation, but the only non-OEM option, and THE ONLY option if you want good r-134a performance. You should be able to fit a pretty big one in a w126. I was restricted to the 14" one in my w123, but it still performs very well.

GregS
'84 300D, 175k
'90 300CE, 165k