I recently tried something on a friends ford Taurus that was pretty plugged up in the heater core.  I put 50% distilled water with 50% distilled white vinegar, let her run it for a few short trips across town, then drained it.  You would not believe the amount of scale that came out. 

Then I filled it with distilled water, ran it up to temp and drained it off 4 times until the scale quite coming out and it was mostly clear.  Seems to have don the trick.