I had a look at the 91 300SL. The actuator is under the intake so I can't get a pic of it. The actuator linkage runs from the throttle body towards the block to the cruise actuator.

The throttle cable attaches to a pivot that appears to have a vacuum hose attached to it, not sure what this does.

The only box I see that might be cruise is HGS 008 545 26 32 it is the type with a locking knob that plugs in like a relay. It measures about 2" deep x 4" wide x 5" tall .