Routing question

[Leon Botes]

I have a box with three interface cards.
inside interface - 192.168.0.0/24
outside interface 1 - 196.15.149.64/26
outside interface 2 - 172.16.254.0/24
Inside connects to lan with windows pc's
outside 1 permanently connected to isp via wan router with diginet (t1 type
line) and is viewable from internet.
inside 2 connects to internet via DSl router using nat , connection ip
cahnges daily.

What i want the box to do is send all outgoing requests that originated from
the box or the inside lan out the outside interface 2 which it does by
setting default route.
However when a connectio originates from internet to the outside 1 interface
it must route its reply back via the same route and not via the outside
interface 2 which it is doing hence making the box invisible to the outside.

PLEASE any suggestions?