client certificate authentications behind SSL VPN

I have an SSL VPN (web portal) set up. I have an IIS server on the backend with a site which must be HTTPS and must require client certificates (x509). It seems like the vpn is working great, but the client certificate is not getting passed along the HTTPS request from IIS to client. We keep getting 403.7 (Client certificate required) errors. How I can bridge the client certificate through the SSL VPN? What I would expect is that when we make a web request that goes through the firewall to the IIS server, that we would get challenged for a client cert for the IIS website (its set to require client certs like it always has) and that client cert information would be passed along the HTTPS request. We have to be able to programatically access the x509 cert through code on the IIS website, thats why we need to have the cert passed along.