If you had a real diagram this may be easier
I'm seeing this, so correct me if I'm wrong:
10.40.45.0/24 -> ASA <---VPN--->Pix --- DMZ
The DMZ is off of the Pix? If so, you'll need to not nat your 10.40.45.0 into the DMZ. The Pix obviously has a route back to your 10.40.45.0 subnet (if you can get to anything?), so you wouldn't need any special routes. If you cannot see your 10.40.45.0 subnet from the Pix, you'll need to point the route to your next hop:
route outside 10.40.45.0 255.255.255.0 Pix's gateway
HTH,
John
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