Thanks everyone. The 24 volt relay was my second choice if there wasn't a better place to get the signal.i will use a relay.
This controller will be for electric trailer brakes. I found the spare wires running from the front to the back. Since they are small, I will parallel two of them. The bus was already set up for air operated supplemental brakes on a toad. ( M&G I think) I plan to rig up a Jeep for flat towing a little later. Am I right thinking that there are are no controllers or lights etc. needed with the M&G system, only the air line from the bus bumper to the unit at the toad master cylinder?
Thanks for the help!