My car has never had a hose behind the dash side vents, only the side vents. I saw on the GT forum that the dash side vents go directly to a hole in the firewall next to the wiper assembly. Is that where they go on the spider as well?
That drain hose is an OEM from Classic Alfa UK. They are a pain in the neck to install through the rubber grommet while keeping rubber in the mounting hole.
One more thing to mention, there is a clip to be installed after vent tube is in place. unfortunately, clips on mine are AWOL. if I had one (which I'm looking for) to picture, I'd post.