once a cooling system gets in a condition where contaminates build up, such as this case, it'a nearly impossible to recover. I have several customers that about every other year we're flushing their heater cores to regain full heat. I've did my 01 pickup a few years back and it's had impeccable service. Heater core is just a convenient place for "gunk" to accumulate. Just beware that flushing using high psi or "bursts" of air can damage a heater core and you might be pulling the dash again.