Like most convertible tops, the BMW Z3 uses steel tension cables that are threaded into the pockets that run along the side windows. You can check your existing cables easily. With the top up and windows open, run your fingers along the edge of the top until you feel the cable inside edge of the top. If it feels tight, then it can most likely be reused when you replace your old convertible top. If it is loose, then it still may just need re-tensioning during the installation of the new top or you may need a new set of spring loaded tension cables.
An installer can also repair damaged tension cables, but if you would like to also replace the tension cables at the same time you replace your convertible top, this is the item you need.
NOTE: We have found that BMW's documented change period from SHORT cables to LONG cables is not accurate. The only way to verify the correct cable length is to partially remove the top and compare the existing side tension cables with the cables listed above.