[bestgenman]

In the cabinet @richard refers to , is a group of wires that run to the front.  there are spares in this bundle, pick one for the brake activation lead.  connect this wire to your controller's brake output.  The power for the controller should be fused (high enough to protect the wire and above the current requirement of the brakes).  There is a ample power up front for your controller.

The lead you select should be as large as possible to compensate for voltage drop.  You may want to parallel two wires for added copper size.

Locate the air brake pressure switch, use this for the activation lead to your controller.  Make sure this lead goes plus 12 when the brakes are pressed.

I am assuming the controller is using an electrical signal rather than air pressure to activate.