Its a massive pain. You have to take your dash out. And make sure you put it back in right, I broke my temperature knob somehow. Its stuck at FULL Heat. Works ok in winter but sucked BALLS in the summer. Want some AC? TOO BAD!
ive seen numerous people who THOUGHT they needed a heater core but when it really just needs to be flushed out from the engine bay. Not saying it will work for you but ive seen LOTS of them fixed at my dads shop when people expected to be paying out the ass for the job. You could try it and see what happens. See if the water comes out FILTY as F!