BBB Industries has free online wiring diagrams. For your vehicle, under Body & Accessories, Trailer Towing, the diagram (WRGC15SILVER102a) clearly show that the Battery positive terminal on the 7 pin trailer connector labeled "A", is "hot at all times".
Coming out of the underhood fuse block this wire is the red with light green wire. Cut this wire and insert the etrailer isolator relay listed above.
Finding a "hot with key on" circuit is another isse.