5ktq heat

[R Bristol]

I replaced the thermostat on the my 87 5ktq
it was stuck closed and I had no heat. Well
the new thermostat is working and there is 
warm -hot fluid in both hoses into what I
presume is the heater core? A large white plastic box 
is the best way I can describe it under the valance.
I replaced the antifreeze and I am still getting cool air thru my heat vents. Looking for
some help here it's getting cold here in MI. 

Thanks,

Randy Bristol
north at bright.net
86 4kcsq
87 5kcstq