Port Forwarding - Squid -

Hi,

I have 2 servers
1. Squid Proxy Server
2. VPN/ FTP BOX

I've use Squid Proxy server external / internet IP to login to VPN/FTP. So, configure my squid proxy server's iptables to port forward to VPN/FTP box.
iptables -t nat -I PREROUTING 1 -i eth0 -p tcp --dport xxxx -j DNAT --to-destination my_vpnftp_internetIP:xxxx
iptables -I INPUT -p tcp --dport xxxx -j ACCEPT

I manage it get it working, when using outside internet (home dsl , cafes etc), but when i am using the internet behind SQUID PROXY SERVER, it doesn't work.

Pls help. ty