source ip based outbound routing & multiple internet links

heres our scenario...

two internet links via different isps. inbound routing is to the different ip subnets
provided by the isps. outbound routing is done by inspecting the source ip.

i want to setup squid in such a way that if one of the internet links goes down the
other will be used.

to do this obviously the source ip address will have to change.
i thought that perhaps one squid proc talking to two parents with source ips destined
for either internet link might work. but how will squid know an internet link is down?

thanx for your help,

colm ennis

-- 
Colm Ennis              | Gradient Solutions
mailto:colm@gradient.ie | http://www.gradient.ie
Voice: +353 1 240 0500  | Ormond House, 12 Lr Leeson St,
Fax:   +353 1 240 0501  | Dublin 2, Ireland