As stated it sounds like one or more of your rear 8 hydraulic cylinders is leaking hydraulic fluid to the ground. Your hydraulic cylinder pump reservoir is probably EMPTY and unable to create the pressure needed to move the latches.I cannot get my rear latches to lock the soft top. I can see the top moving down but when I release the button it pops up again or when I hold it for a little while it pops up.
I also noticed hydraulic fluid on the ground when I hold the button there are two little ports one on each side of the car.
Is this normal to have it come out when you hold the red button is it an overflow or does that mean I have a problem with the hydraulic pump?
If you need to move the top you will need to fill the reservoir then move the top to where you want it until you can repair the cylinders.
This is the first time that I have heard of a 1998 needing it's rear cylinders rebuilt. The two windshield cylinders are usually the first to go and this will give you a heads up as to when you need to repair the rear cylinders.
You will probably want to DIY because of the Indy or Dealer cost. If so, please read http://www.benzworld.org/forums/r129-sl-class/1307426-r129-soft-top-hydraulic-system-ultimate.html and feel free to contact Dennis Ficken for rebuilt cylinders.