Thermostat housing leaking again!?

[FatSean]

01 ZX3 with the Zetec. Last year I got the expected cracked thermostat housing leak. Bought new ford housing and thermostat, replaced it, torqued the bolts correctly. Housing came with a new o-ring that goes between the head and the housing.

Now it's leaking again! Same weirdness...it'll go days without losing a drop and then one day you'll smell it as you drive, stop, and the thing is 2 quarts down.

Did I do something wrong? I heard about a 'plug' with it's own o-ring that I may not have replaced the first time...can you see that bit with the car assembled?

I expect I'll have to stay up late and take the thing apart again, but I was hoping you guys had some tips.

Thanks!