[yetibone]

The trick is to fill the core up with water first, then blow it out (from outlet side) with 90psi compressed air. Do this about a dozen times and see if that helps unplug the coolant passages. I've unstopped many a heater core that way, without damaging the core itself. Mostly 3.0 OHV Taurus' and 4.6 F150s' that were factory filled with G-05 antifreeze. I did fix a Wangler that had been overdosed with Bars Leaks this way too.

Water from your household spigot doesn't have the force necessary to move obstructions out of the flutes in the core. The Jeep's own water pump prolly has more power than a residential well pump, or a city tap at the meter. Air pressure has the force to do it, but no surface area to push with. Filling the core up with water (outlet side first) provides a medium to push with, and air gives a means to do the pushing.

Whatever you do, DO NOT USE DRAIN-O. The water pump, and T-stat housing wouldn't survive unless the residual acids left behind in the heater were neutralized before returning the system to regular use.