Your battery should have venting hoses attached to the hole in the center. MB did not do this as standard on the 98 ml, which I have, & probably not on the 99 you have. You can check with the dealer or any store that sells replacement batteries.
87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)