You need to replace the mast. Don't worry about water going down the tube as there is a drain tube at the bottom of the unit designed to take care of this. The mast is ~$30+/- at Nissan dealers. If any of the nylon gear is broken off (you'll see this when you remove the old one) you have to remove the entire unit from the trunk, open up the reel enclosure and remove the broken piece. If the gear's not broke, simply remove the chrome collar nut at the base of the antenna, turn the radio on, pull the mast completely out, slip the new gear back into the hole, turn the radio OFF and feed the gear into the unit. Reattach the collar nut and yer in bidness!
Chris
90 & 94 GXE's