Yesterday I saw a post regarding this problem but I'll be darned if I can find it now. Anyhow, just below the battery you'll find a 3/4" O.D. tube protruding through the firewall. That is the a/c condensate drain. At highway speeds running the a/c, air pressure under the hood can force the drain to flow back into the cab. My solution was to attach a rubber elbow with a hose clamp to this drain tube. Now we run high and dry. <img src="/forums/images/graemlins/cheers.gif" alt="" />