In that case, take the ground from the 2600 I mentioned eariler, and find a ground point on the hip screen (if there is some shielding somewhere, that's usually a good ground point) Then, just connect the 2 points together.
As for the battery, I'm not sure how it works on a junior... you may want to use a new switch and wire it directly to the 7805. If you don't mind using a different switch, I can go into more detail