I know this is an old thread but it has been resurrected because so many Ram owners in the 2013's and up have no heat.
Here's my findings on where my troubles were:
In 2013, Ram switched to OAT coolant, some prior years up to 2012 used HOAT. There is such a difference in the two, they cause gelling in the system. If your truck calls for OAT, you'll have to order it or go to the dealership. Parts stores have nothing compatible.
So, if you have ever had your coolant system topped off or like me, I had a water pump recall and the dealership replaced what was lost with HOAT, you have gelled slime in your heater core.
Here's what I did:
I disconnected my heater core lines under the hood. One at the firewall and the other at the intake. It's the squeeze type clamps. I purchased some fittings and a small piece of heater hose. I did nothing fancy, I just used electrical tape to tape and seal a fitting to my water hose and air compressor hose nozzles. Then I started blasting the core with the water hose in both directions. Full pressure, several times. After a while, I started using air blasts in both directions. I filled the core with water then blasted it out with air in both directions. On the last air blast, I refilled the core with a jug of distilled water. I put my truck back together and have had great heat ever since.