2000 chevy S 10 heater problem

I have a problem and cant figure it out. My 2000 chevy S 10's heater does
not blow hot air when control is in hot or warm position. I have had it
apart and it has 3 plugs in the back. I hear no noise as thought it may be
a vac leak.

Any suggestions of what the problem is or how and where to look to fix
it?

Thanks