1998 GMC C1500 heater problems

My name is Josh and I am having a heater problem.
when I turn on my heater it only blows out of my defrost vent it will not turn hot not even luke warm.
my heater core it hot and so are both of my tubes going into the truck through the fire wall. My radiator and the aux is full, but my switchs are also not switching from floor to front it only stays on defrost vent and only blows cold I have no idea what to look at to fix this.

Thank You,
Josh