I am doing the retrofit now to my MY98. There is a good document on RennTech and I'll post it here. When I'm done I'll post my experience here.
UPDATE - here is a link to the thread on renntech. The document is at the end.
I'm ready to complete my install except I'm missing some connector pins.
Adding Cruise Control to a MY98 - 986 Series (Boxster, Boxster S) - RennTech.org Forums