1995 Mercury Villager Only blows heat while excellerating

I already replaced the thermostat and the coolant level is ok.
Someone said that there's a little door that has to open to let the air through and maybe that's the problem. I don't know.
Thanks,
al

I have the same type of problem with my 98 Villager. If I hold the engine at 3000rpm the rear hearter works as well. Could our problem be air in the system? Would "bleeding cooling system" help?

I dusted off my Haynes Repair manual for 93-01 Villager.
Chapter 1-23 talks about refilling the system after
servicing the cooling system. It goes step-by-step
on bleeding the system.
It takes a little time but it worked!
Setting still in the driveway and I got hot air front
and back!
Grace,
Andrew