Hi,
First thing that you need to have in mind is that Cisco does not support active-passive with WCCP but you can make it work anyway.
If remember correctly WSG uses service 0 for HTTP-WCCP communication, service 0 has the highest priority for any Cisco WCCP capable
device so what you need to do is just configure your standby WSG with a higher service number (make sure you don't use one of the protocol
defaults, i.e 70 = HTTPS).
So on your 6500 you should have something like this:
ip wccp 0 (primary)
ip wccp 10 (backup)
Then you need to configure both service under the same L3 interface or SVI whatever your case is
ip wccp 0 redirect in| out
ip wccp 10 redirect in | out
That's it, if 3 WCCP "Hello" messages get lost between the 6500 and the WSG then service 10 will be used to redirect client traffic.
HTH
__ __
Pablo