Mixed/contaminated coolant can clog up cores on the gassers, too. I just flushed one on a 2015 2.slo Jetta that had been previously messed up by morons. So yeah, it can happen on those.
I have been reasonably successful with flushing them the opposite direction of flow with a garden hose after soaking them with Permatex Heavy Duty radiator cleaner.
That stuff is powerful, and it works. I *JUST* put it in the heater core/pipes, and not anywhere else. Yes, I know it says not safe for aluminum, and yes I know the pipes and core are made out of aluminum. I don't leave it in there for very long, and if it messed something up (never has, but if) you'd be back to replacing it anyway, so what is the harm in trying?
Here is how I do it:
Disconnect both quick couplers at the heater core pipes on the firewall, and move them out of the way and secure them so nothing else gets in them.
Get two lengths of bulk hose, about a foot or so long, 3/4"/19mm ID, and slip them on to each pipe, and angle them UP.
Pour the cleaner down both hoses until they are both full. If the core is not completely clogged shut, which they usually are not, just pouring it in one hose is enough, as liquid seeks its own level and it will come back up the other. Try not to spill any. One quart bottle is enough to do it a couple times at least.
Let it soak for about 20 minutes. You may see some bubbles and foam come up as it dissolves the solids inside. Pull the hose off from the TOP pipe connection, leave the lower one in place.
Rig up a garden hose to fit in the hose that is still attached to the BOTTOM pipe, then turn it on full while watching what comes out of that top connection. You'll see a black/brown slurry of filth come pouring out... keep the hose running until it comes out clean and clear.
Turn the water off, remove the hose, reconnect the quick couplers, refill the system with G12 mix, put anything back together you had to remove to get at the firewall, take the car for a drive and make sure the system is full.
