After help from rkosiorek, I drew up with this diagram:

I matched that knowledge with the pinouts of the throttle control of my EBikeKit, and came up with this:

It's hard to tell, but I spliced the +5V, GND, and THROTTLE wires in two, and added connectors to both ends. This was to allow the cruise control to be plugged between the throttle and controller without actually modifying the plugs on the throttle or the controller. Notice that the orange wire, BRAKE, is NOT connected, and doesn't have to be unless you want to use it. Cruise control can be turned on and off via the "cruise" button.