can you manually lower roll bar on a 1999 500sl

NO! It's a roll bar! If it moved at all, and you rolled it, the whole thing could come down on you. How long have you had this car? Did the roll bar come with it? I think the only thing you can do if it is a problem would be to remove it. If you are lucky, it is bolted in and not welded.

IT RUNS OFF YOUR TOP MOTOR LOCATED UNDER A COVER UNDER YOUR SPARE TIRE. IF THE TOP MOTOR OIL TANK IS EMPTY IT WILL NOT WORK. YOU MIGHT HAVE A BLOWN FUSE. YOU CAN NOT PUT THE BAR DOWN MAN BY HAND.

Sorry, I thought you had a real roll bar and not a trim item.

The Owner's Manual for my 2001 SL500 says, "The roll bar can be lowered again after an automatic deployment by pressing the upper half of the roll bar switch (for at least 5 seconds) until the roll bar drive mechanism audibly engages. Then press the lower half of the switch to lower the roll bar". If you have not tried this, I suggest that you do. The Owner's Manual also mentions that during both the raising and lowering of the soft top one of the first steps performed (automatically) after moving the switch for the soft top is, "Roll bar lowers". I am guessing that until the roll bar is lowered, there is a safety interlock that will not allow your soft top to go either up or down. Get the roll bar down and I suspect that the soft top will then function properly. Good luck.