If you remove the circlip that holds the plastic part in you'll find there is an O-ring in there that has probably gotten hard. Your tugging just loosened up the old seal so now it's leaking. You might be able to source a new O-ring at a good hardware store but it's probably metric so the dealer would be more certain. In a pinch you can just bypass the heater until you get the parts you need.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold