'98 V6 tacoma:
i changed my clutch on my tacoma recently. to get to the top bolts i unbolted and moved the heater valve off the firewall. since then, the cable to move the valve to hot/cold is stuck @ 2/3's what it's supposed to and the heater only has a 1/3 the heat it normally does. i've taken off the cable and turned the valve to both extremes, both of which blow even colder air. with the cable on, i get the "warmest" 1/3 air. the vents still blow at full capacity.

so bad valve? or bad heater core?

 

I recently had this problem too. Turned out to be the cable that goes from the bottom of the cold/hot knob to the right, behind the glove box, under the golve box and back behind the center console at the bottome near the firewall. There is a door there that allows the hot air into the vent tubes (you can reach back there and manually flip the door open and closed to verify if this is the problem). My cable was bound up at the knob and eventually came off as I was forcing it to turn. Had to take the center dash parts off, unscrew the control box and re-attach the cable but now it works perfect!

 

All I know is a bad heater core usually just equates to coolant leaking. So, its probably something else.