The turn signal on my Boxster didn't self-cancel when I bought it. The problem was that the ends of the delicate tabs that actuate the self-canceling mechanism had broken off. My fix was to replace the broken clockspring and the attached combination switch with a used clockspring/combination switch that I found on eBay. This has worked just fine for years even though one of the tabs on the replacement was broken when I got it. It's hard for me to understand why Porsche would have designed the part to be so fragile... Anyway, here's a photo to show what I'm taking about.
A few photos from when I did the work here -
https://rx19jones.wixsite.com/projectboxster/projectboxster-1
dj