Here is the bulletin:
http://www.bmwtis.net/tsb/bulletins/bul ... 30503g.htm
If you have had your Z4's third brake light replaced due to the crack, I'd recommend taking a look at the springs to make sure they used the updated version. My dealer only swapped two of the springs, and put the washers between the nut and springs (supposed to be between the trunk [boot] lid and springs), so don't be surprised if your Z4 still has the old springs. And if they do it could lead to the third brake light cracking again, likely when you're out of warranty.
Here is a pic of the old and new spring with washer:
The old springs (left) are about 1 1/8" long with about a 5/8" diameter larger end. The new springs (right) are about 1" long with a half inch diameter on the larger end.
When installed, the spring's larger diameter end faces the the trunk (boot) lid (as shown in the last image in the link above). The new spring's smaller diameter end will pass through the trunk lid hole, and so the washer is needed.
You can check your Z4 to make sure it has the correct springs by removing the black plastic inner trunk lid handle (left side of trunk lid). It's held in place by two Phillips screws, use care removing it as it has a couple of tabs holding it place. Once removed, the springs will be visible and you should be able to see the black washer between the spring and trunk lid. If you do not see the black washer, you may not have the new springs. To remove the nuts you'll need a deep 8mm socket, to be safe, I'd recommend loosening each of the three nuts a little at a time until there is no more spring tension. It goes without saying that once they nuts are removed the third brake light will not be secured and can fall out if you close the trunk.
