Redirect OpenVPN gateway traffic to Privoxy

Question

I would like to redirect the traffic that goes through my OpenVPN gateway to Privoxy, like so:

OpenVPN client->Internet->OpenVPN gateway->Privoxy->Internet->Webserver (and vice versa)

So far I managed to get the OpenVPN gateway working fine. Connecting to the Privoxy proxy appears to be working aswell. But redirecting port 80 traffic via iptables just won't work (external webservers are unreachable). What would my iptables config file have to look like?

Thanks in advance

PS: This is what I came up with so far...

iptables -t nat -A POSTROUTING -o eth0 -s 10.10.10.0/24 -j MASQUERADE

iptables -I FORWARD -i tap0 -j ACCEPT
iptables -I FORWARD -o tap0 -j ACCEPT

# No success with the following line
iptables -t nat -A PREROUTING -i tap0 -p tcp --dport 80 -j REDIRECT --to-port 8118