Both my '05 Arc and my wife's '04 Arc wagons started slowly leaking coolant at about the same time. Her car (The Mighty Thor) started leaking about a week before mine but it's due to go into the garage for a series of much needed repairs anyway so I just kept adding water to it. When mine (The Muttmobile) started leaking at about the same rate I guessed that it must be the heater control valve. I have been through this before with my '03 Linear and '03 Aero. After pulling the cover off the wiring connector and probing around between the engine and firewall, my suspicions were confirmed. I ordered a pair of the valves from eEuroparts. However I drive quite a bit more than the wife and the new parts will probably not be here before the end of the week so I decided to try pulling the offending part off and attempting to plug the leak. I seem to remember trying this before with little success. I decided to try harder this time. The last time I simply squished some five minute epoxy around the part that seems to be press-fitted into the end - the part that the bottom hose connects to. This is the only place it can leak. There are no open seams or cracks anywhere else. I used liquid epoxy made by J B Weld formulated for plastics. I cleaned it with alcohol and blew it dry with compressed air - twice. Then I used sand paper and a wire wheel to rough up the surface. Once the liquid epoxy set, I applied epoxy putty - again, made by J B Weld: "Water Weld". It's formulated to be used on plumbing.
I let the epoxys set up overnight and I'll be putting it back on in a few minutes. My plan is to leave it on as long as possible to see how long this repair holds while keeping the new part in the glove compartment just in case.