Most likely it's leaking from around your coolant cap - it's a pretty common problem.
When the cap doesn't seat tight, it allows coolant to seep up around the cap where it pools in the "catch basin" where your oil and coolant cap live. From that point there is a drain tube that runs down and "pees" the coolant out near the RR tire.
Best thing to do is first - make sure your cap is on correctly and tight. It's easy to not have it on correctly. Go ahead and take it off and re-install, making sure it threads on correctly and you have it screwed down completely. If that doesn't solve the problem, get a new cap. But get the good one - Porsche has updated the cap - it's blue. Don't get the black one.
Coolant Cap:Suncoast Porsche Parts & Accessories
More than likely this will solve the problem.
2002 Porsche Boxter S
2001 Porsche Boxster S
1999 Porsche Boxster
1999 Porsche Boxster
1999 Porsche Boxster
1999 Porsche Boxster
2000 Porsche Boxster
1999 Porsche 996 C4
1972 Porsche 914
1984 Porsche 928S
1984 Porsche 928S
1985 BMW 635 Csi
Threaded Mode
