Backup WAN interface vpn tunnel

I have a client with a Juniper SSG 550 latest version Screen OS that wants to send all his VPN traffic using his backup ISP connection. He will use his main ISP for web traffic. I know I can send traffic to the bavkup WAN via souce routing. My question is, How do I make my backup WAN ip available from the internet to set up a VPN tunnel. Is there a special route to make my backup WAN ip available from the internet?


Thanks