Here's one source: workhorseparts.com
Nearly all those mystery modules have a P/N either on a stick on tag, or embossed into the plastic. Workhorse has many modules listed as a Modulator...mostly for the ABS system. Modulation is a term applied to a circuit that rapidly switches something Off/On. I would be surprised if any of your lights were 'dim-able', which would be the only reason to 'modulate' them. Only thing I can think of that would be dim-able would be the dash lights. If you have a simple pot instead of a heavy rheostat to dim your dash lights perhaps that's what it is...in which case you could bypass that whole box for the lights, and install a rheostat for the dimming function. (Just thinking out loud here).
Maybe if you posted a picture of the device we could give you more specific help.
Here's an item called an Electric Modulator: ELECTRIC MODULATOR 12 V DC
but they don't give any details.