This site http://gdl-online.com/begin1.html#contents
has a number of interesting articles on cruise controls, how to locate the components, how it all works, and rebuilds them for you as well with a 4 year to lifetime warranty (See specials section).
The factory CD rom has a lot of info, but is a bit hard to use from what I hear. I don't know if your 90 came with all it's books, but my 87 has a book that basically shows exploded diagrams of most of the car's components and assembly that is useful. Also, www.************************
will let you fill out an online catalog request - the catalogs for W124s used to have exploded diagrams with part names and prices that was helpful on several occasions. Ebay also is a source for missing manuals, and occasionally for tech shop manuals on your car. (Those coupes are great looking cars!)
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E