IPtables Port Foprwarding

I'm battling a firewall issue. Machine "A" can not see machine "B", so I want to bounce through machine "C". Unfortunately, it has to be a selective redirect, as "A" needs services from "C", as well. Three steps, executed on machine "C":

[root@c]# echo 1 /proc/sys/net/ipv4/ip_forward
[root@c]# iptables -A PREROUTING -t nat -i eth0 -p tcp
    --dport 3389 -j DNAT --to 10.11.12.13:9833
[root@c]$ iptables -A FORWARD -p tcp -m state --state NEW
    --dport 3389 -j ACCEPT

First step enables forwarding. Second step takes all packets for port 3389 (machine "C" doesn't run MS terminal services), sends them to 10.11.12.13, and twists them to 9833. Thirds step fires them back out.