I have a 2013 with a similar issue.. Lukewarm air coming from the furthest left vent, and each vent to the right gets slightly warmer. The far right vent gets pretty hot, about what I would expect from all the vents. I originally lived in Phoenix when I bought the jeep new and never really needed the heat so I can't comment on how long its been like this, but now that I live in WA it's definitely noticeable. I have the auto climate control, so when it's on that setting it never blows through the main vents, only the floor and windshield. So the issue is only noticeable if I change the setting to the main vents, which leads me to believe it could be a blend door issue. However when I first bought the jeep I had that running water sound coming from the heater core area but that resolved itself, and I had a good amount of casting sand in the reservoir after about 30k miles and never took it to the dealer. So I can't really determine what the exact cause is, and it's long out of warranty now so I'm not going to do anything about it until there's no heat at all. Let us know what ends up happening.