My mothers rear window washer was not working for the past few years. During a visit I did some investigating and found the washer motor worked fine but the fluid would leak from the transverse frame rail adjacent to the lift gate. I carefully popped off the carpet molding at the aft end of the bed and found a small opening to the frame rail. In this approx 2" x 6" opening were a few electrical connectors and the tubing for the rear window washer. I was surprised and disappointed to learn that the rear window washer tubing is run under the interior carpet. Fortunately the leak I was experiencing was due to the rubber elbow that connects the check valve to the tube that runs into the tailgate had pulled loose. I tried to reconnect but could see that the two ends were barely long enough to make a reliable connection. Now the light bulb goes off. A few years back the rear window was replaced by a Chevy dealer due to a break-in. I suspect when the window was replaced they may have disrupted the original tubing routing in the gate leaving it a bit short for making the connection to the rubber elbow in the well of the frame rail. Anyway the fix was to trot down to the local auto supply and buy 6" of tubing and create a loop in the connection between the check valve and the tubing that goes into the tail gate.

 

Mine had the exact same problem although my rear tailgate and window are still original. The tubing connector was an L shape and it seems that it should have just been a straight connector instead. I was able to pull the tubing together and used a ziptie to hold it together but I may add a piece of hose like you did and create a better connection that's not so tight.