If I had access to my Boxster right now I'd do a quick photo how-to. However, if I can get over to my dad's garage this week (if you don't have the parts by then) I'll make one.
The hard part is the upper portion. A small wire clip pivots on the top end of the cable to keep it secure to the socket on the bottom roof bow. Using needle-nose pliers makes quicker work of this. It can be a little difficult since the roof has to be pulled up into service mode a bit to get enough vision and room to work. Once the new one is on and secured, feed it through the pocket on the weather flap and pop the bottom end onto the lower socket.