I've been successful by using a Monokote heat gun and a lot of patience.  It takes a long time (15 minutes?) to gradually get the end cap up to temp so that it will move. Putting a drill bit of the matching size into the outlet will allow you to put pressure in the joint and feel when it is getting ready to come loose. I removed the cap and cleaned off the mating surfaces so that none of the cooked adhesive remained, then re-glued it with JB Weld.