There is a site that has been posted serveral time which has the how-to for testing purposes. Basically helps you narrow it down without expensive equipment, but I can't recall the site right now.
If it accelerates with the stalk, but won't hold, the common fault is in the cruise amp. A couple choices here - open it up and resolder every joint in the amp (about 300 or so it seems), send off the cruise amp for a reman replacement (as long as the old one hasn't been opened up and DIY fixed), or what I did... go to a salvage yard and pull about 5 cruise amps from junkers and keep swapping amps until the dang thing works.
But before doing all that, I would do a search on cruise amps, try to find the mentioned site and test the switch and acuator before yanking the amp.
Never mind the search Bobby, just looked in my history and found the link to GDL
Hope this helps.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.