Before doing all this check your brake light switch at brake pedal. It might be sending very low current which in turn will disable cruise control.
I had same problem with w124 and after replacing switch from ebay($12) my cruise is working good.
On w126 on other hand it's not working so i have to get amp from jy.

Remember one thing:
Before assuming extreme check simple and cheap things first respectively and if it's not it go for complex and pricey procedures.

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1) '04 S500 Gold/Beige......120k Miles
2) '93 300E 2.8...Green/Tan MBTex..........202k Miles(and going)
3) '89 420SEL...Light Blue/gray Leather....155k Miles (Retired )