It is part of a brake light relay
Roadmaster Brake-Lite Relay Kit for Towed Vehicles Roadmaster Tow Bar Braking Systems RM-88400
It is necessary IF when you have your ignition switch OFF and you still have brake lights when the brake peddle is pushed.
According to what I have read, Its primary purpose it to prevent the toad's brake lights from over riding the brake light signal from the MH. When relay is not used and you apply brakes in the MH and at the same time have a turn signal on the toad's brake peddle will then cause the toad's brake system lights to over ride the MH's turn signal.
Again, that is what I have read and that doesn't seem to totally ring true when the toad has a separate turn signal bulb so this might be mostly true for toads with combined brake and turn signal bulbs.
Don, Sandee & GSD Zeus. Gardian Wings earned are GSDs Gunny (7/11/15) & Thor (5/5/15)
2006 DSDP 4320, Spartan MM IFS, 2013 CR-V, Blue Ox Avail, SMI AF1, Samsung 197R Refer.