My husband & I love Enchanted Rock - and I know the cave of which you speak. It is shown on the map you get at the Park entry. I would think the park rangers could give you the GPS for it. I also found the following about it:
Along the northwest face of Enchanted Rock, near the summit is Enchanted Rock Cave. Actually a capped crevice over 600 feet long with some 20 entrances, it is one of the largest caves formed within an inselberg mass. Although exploration of the cave is permitted it should not be done without adequate equipment. The absolute darkness and vertical drops near the lower levels of the cave make it very hazardous for amateurs. Formerly the nesting place for rock and canyon wrens, and a roosting site for cave myotis and other bats. Enchanted Rock Cave is one of the most ecologically damaged areas in the park.
http://www.texfiles.com/erockhistory
Personal experiences of hiking ER & several mention a cave or caves.
http://www.trails.com/trail_reviews....lid=HGS131-030
One guy’s experience w/ the cave
http://www.dailyepiphany.net/2000/aug/25.htm
Have Fun!