OK thanks for EPC tip, I logged in and can see p/n different for the amp. For 124 it lists 5 different ones, for 87 SDL it lists only 1 and says NLA.
Also shows location of cruise amp different in SDL than what Im used to.
Which leads to next question - where is best place to get W126 manual ?
I see one here -
http://www.mercedesmanuals.com/PhotoGallery.asp?ProductCode=W126
Ive also heard of buying from MB but I was at dealer today and the parts guy had no clue.