Ahhh... I have just recently replaced the front trunk cable. I was unable to get the old outer cable out (it routes tightly behind the fusebox and relay panel) so I just made a new "inner" cable out of 3/64" 1x19 wire rope and swaged new ends on. I got the tools from amazon.
However, I believe the rear trunk cable may have a more "friendly" routing
If you can find the Porsche service manuals (they are available for download somewhere; I actually bought a CD of them from eBay from England or somewhere) it shows you the routing, and also the steps for removing the handle assembly in the car so you can get the new ends on, and the steps for working with the trunk latch assembly.
I got my new hood cable from a user here: itsnotanova. If you need a new rear trunk cable assembly, Woody can hook you up and also give you tips on how to get the old cable out. He is apparently much more flexible than me because I couldn't get my old one out and there is no way in #$%^ I could have gotten the new one in 9hence my method of just replacing the inner wire rope portion, leaving the existing outer portion intact)
I started with the inner handle portion first, fastened it on, and then threaded the wire rope through the old housing. I used some plasti-dip to mark where the cable should terminate (and still got it wrong) - my advice would be to leave a little extra room on that end if you do it that way, put a couple of loose fasteners and then swage one on then end; that way you can remove them to adjust.
Or maybe adapt part of the adjuster from a bicycle cable like the folks do that fix the window regulators.
I am super busy with work today but I can chat some tonight if you would like to hear more about what I did. I will PM you my contact info.
Steve