Problem with external proxy (squid) authentication

Hy all.
My setup is:

I setted up a transparent proxy on ROS 3.0rc13 on x86:

[admin@xxxx] > /ip proxy print
 enabled: yes
 src-address: 0.0.0.0
 port: 8000
 parent-proxy: 192.168.0.89
 parent-proxy-port: 3128
 cache-drive: system
 cache-administrator: "xxx@yyy.it"
 max-cache-size: none
 cache-on-disk: no
 max-client-connections: 1000
 max-server-connections: 1000
 max-fresh-time: 3d
 serialize-connections: no
 always-from-cache: no
 cache-hit-dscp: 4

and then i set a redirect:

/ip firewall nat print
2   ;;; Proxy Redirect
     chain=dstnat action=redirect to-ports=8000 src-address=192.168.0.91 
     dst-address=!192.168.0.89 dst-port=80 protocol=tcp

The only thing i obtain, is a “cache-denied” from squid.
I use radius to auth my user with squid, and if i set it directly on the client’s browser, it work perfectly (popup request with user and pass to client on first attempt to access to Internet).
It seems that RoS tryes to access my squid without auth. How can i force it?
I tryied to set a dst-nat/dst-nat, but it, obviously, won’t work. It seems that i need for transparent proxy (but with auth).

How can i do?

Thanks

No one?