Heater doesn't work. 2000 GMC sonoma

I have a 2000 GMC Sonoma ZR2. I tried running the heater for the first time in months since it's going to be cold here in Minnesota soon! When I turn the heat on it just blows cold air and makes a hissing sound that goes in an out inside the drivers side dash. Even when I turn the car off you will here it hiss for a few more seconds. I've replaced the thermostat and I haven't flushed the coolant yet. Any other ideas would be greatly appreciated.

Thanks!
Matt

I think that hissing sound is a vacuum hose since the vents have trouble switching from the floor to the upper ones. Sometimes the vents won't change and air will slightly come out the last ones that were selected on the dial. Would those vacuum hoses affect the heat at all?

More than likely. If the actuator isn't switching the blend air door from one position to the heater position, then the heat from the heater core wont be directed to any of the vents. Maybe the hissing is a vacuum line cracked or broken. though usually when vacuum to the selector switch is gone, the a/c or heat only blows through the top windshield vents and nowhere else as that is the default position...though I could be mistaken.

Did you blow your heater core? I'd check, coolant would be everywhere. Other than that i would just blow out your coolant, before ripping out your dash.
or check your vacume hoses