99' 4wd 4door 170k

Just finished the heater core change out. I purchased the blazer about three months ago and the guy I had purchased it from had the core bypassed.. Being in Middle Tennessee I haven't even had a chance to turn it on to even need heat. Question. I have hooked up the hoses to the core and no where in line did I see anything that resembled a control valve. I know on other cars that I have had in the past, there was an electric or vac line that opened and closed to allow the flow of hot coolant through the core. The system in my truck is the three ***** Fan speed, temp, selector switch. I haven't removed the bypass yet because I do not what to sweat my rear off driving down the road as of yet... hehehe

 

The heater core flows full all of the time. There are doors in the HVAC housing that direct air over the heater core when heat is requested and block it off when it is not.