Depending on the installed low voltage control panel, the step activation relay may be intergrated within the control box / charger unit. The D+ signal (as the last poster discussed) activates the step relay amongst things and you're best bet is to get hold of a wiring diagram for your MH which will point you in the right direction as to its location. There well may be a designated fuse protecting the step motor which is worthwhile checking out after you have found it!