Firewall pix behind router

Hello,

I'm going trying connect 2 pix in a Lan-to-Lan vpn in the next schema:

Lan1 -- Pix1 -- Router ------- Internet --------- Pix2 --- Lan2

The problem is that between Pix1 and Router there is a private lan and although I have redirected all traffic get by Router to Pix1 (ipsec, ip 50, 51,... included), the vpn doesn't work. Any idea?

Thanks in advance,

J.