Site-to-Site VPN

Tring to do a site-to-site VPN between Cisco-PIX and ‘Back Firewall ISA”

Situation:
Site-A———Internet——————Site-B
LAN—PIX——Internet——Router—ISA—LAN

Site-A side shouldn?t be a problem, however the Site-B is going to be a challenge because the VPN services of ISA cannot be published thus the site-to-site VPN will end on the inside-IP of the router. I tried forwarding IPSec from router to ISA and configuring site-to-site VPN in ISA with no luck.