I'm not getting very good heat through the vents

See if both heater hoses are getting too hot to hold going to your heater core at the firewall?

Thats the first thing I always check stop leak can clog up a heater core you must make sure both those hoses are getting too hot to hold. If the blend door works then all that is really left is a restricted heater core.

Do you get a sufficient amout of air blowing out of the vents, even if it is cold?

Aright if the heater hoses are too hot to handle your going to have to look the blend door again because if the hot coolant is flowing thru the heater core then the blend door is the next thing to be looking at.

I am with saturntech9, thats where you should go next.

Gentle men, we are assuming that the heater core is COMPLETELY clean. It may be partially blocked, and only part of the heater core is getting anything at all.

I know you have said that you have flushed the heater core, but have you back flushed it.

Alot of times the crud will not be displaced with regular direction pressure because the path of least resistance has already been created.

I suggest removing the firewal hoses since we know the hot coolant is reaching that point. Then use moderate pressure, feed both orifices with pressure and see if anything else evacuates.

If there is still a problem, then I would suggest your heater core is in need of a replacement.
I would not suggest this lightly as most core are a female dog to replace.

Good luck.

If both hoses are getting too hot to hold then there is some pretty good flow thru the heater core. If you back flush it like Robert says you should be able to see the flow or lack there of it. But in my expernice if both hoses are getting too hot to hold you should be getting some heat inside.

Your welcome that's what were here for.