Routing failover in a firewall filter

We have an SRX300 with 2 completely independent ISP connections. Default route preference handles most outbound connections just fine. However, one ISP is cheap/fast cable broadband where we want all of our web-surfing activity to go. The other connection is reserved for more formal business needs.

The connections are set up with separate routing instances and rib groups in what I think is a fairly standard configuration. But for the web surfing traffic from our proxy server, I have a firewal filter as follows

firewall {
    filter PROXY-FILTER {
        term PROXY-DEFAULT-ROUTE {
            from {
                source-address {
                    192.168.4.85/32;
                }
            }
            then {
                routing-instance VR-SPECTRUM;
            }
        }
        term DEFAULT {
            then accept;
        }
    }
}

Thsi works great, all the traffic from the proxy server goes out the cable broadband. However when that connection drops, the proxy server can't reach the Internet. How can I add failover to this filter?

Sorry if this is stupid-simple, but I'm new to Junos as we are replacing our old SSG routers. I haven't been able to find the solution. Thanks.