There should be a short connector coming out of a wiring loom just left of center on the back of the cab.  It is the harness that runs to the transmission/hydraulic controller. You will also need to T-into the four pin CAN implement harness at the implement connector on the back of the tractor.  (We spliced in a connector closer to the back of the cab so that we didn't have to buy the extension harness). We just cable tie our I-grade controller to the underside of the step behind the cab so that it is easy to move from tractor to tractor.  Make sure you plug/unplug the controller with the tractor shut off.