The cruise control amplifier and the control module (smaller aluminum box) are attached to a gold anodized bracket that attaches to a black metal strut. You want to unbolt the bracket from the strut, disconnecting both boxes with it (you'll see why; you can't get to the rear screw to the CC amp w/o removing the bracket). It is a 10 mm. hex head that attaches the bracket. On many, the "nut" is part of the bracket and does not turn. The 10mm. hex head bolt is on the other side in a recess that requires pushing on a rubber "accordian" to access. Feel with your "pinkie" (deep) and you'll find it. A deep socket helps to keep alignment on it as you fight the "accordian".
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)