Likely heater core. I had same problem with 1996 S-10 with 4.3L.
check heater fan to see if it is working.
if this system uses dex-cool flush the system
it wont blow heat itll blow cold air
my heater fan in front of my 2004 chevy venture van wont work
Your heater core is likely bad
Check your coolant. Is it thick and gunky?
Yes but first make sure that you have both power and ground going to the blower motor...........
I've got a better question, why is this question in the Chevy venture forum ?
modual
Low coolant? Thermostat not opening? Plugged or restricted heater core? Heater core airbound? Heater valve control cable detached or broken?
check you antifreze level if it is low you wont have eat or if you get lots of fog your heater core is bad
fan motor could be bad