PIX routing for site-to-site VPN

Hi,

I got a complicated network as the following:

Site A --- Site B (Fw ) ---- Internet Access ( 10M Broadband line ) Site B (R ) ---- Site C ( IPLC Line )

1. A has an independent network with a PIX506E
2. B has a PIX515E PIX for Internet access
3. B also has a IPLC line to C
4. C is the network resource in remote site

Now that, site A & site B ( by using the Broadband line ) set up a site-to-site VPN between them, so that A can access the network resource of Site B through the VPN connection. site B & Site C set up a dedicated line through Routers, so that whenever B gets the network resources of site C, the packets will be routed to site C; otherwise the packets from B will go to the Broadband line for accessing the Internet.

How can I config Site B ( or the VPN ) so that site A users can get the network resources from site C ?

Thank you very much for your help.

Benson