The full load switch would be part of the throttle switch. Not a lot of fun to replace. As the wiring is easy to get to and a switch can only do a couple things wrong, I would make it look open and see if it is the problem. Since both open and closed are acceptable positions the fault would have to be from a closed full throttlesignal while the idle switch is also closed.
You can try this with a new switch but it sure would be easier to do at the easy to access three wire connector just forward of the air horn.
Bosch Master, ASE Master, L1
33 years MB technician