4th gen Rams for years had an embossed blank on the HVAC box that could be upgraded by carefully cutting along the perimeter to add a cabin filter and a snap-in access door from the minivan. The 2015 Laramie Longhorn that I bought from my elderly neighbor was almost fully loaded but did not have that cabin filter, so I snagged a kit off eBay and made the magic happen.
I have replaced 2 heater cores on Ram 2500s, and it is a chore as the airbags have to be disabled, the steering column is dropped, and the dash is removed as an assembly...easier said than done and with a helper, but I managed to do both by myself out of necessity.
2nd gen Rams had an issue with a plastic adapter between the blend door motor and the blend door shaft. This adapter would develop a split so that the blend door did not rotate fully to completely seal off the heater core or the evaporator. Not easy to check unless HVAC box is disassembled. The symptoms would be heater that does not get very hot and AC that does not get very cold, yet engine coolant would be at operating temperature and AC pressure would be good. The QuadCab had this problem for years but Dad didn't say anything about it until I started driving that truck and asked...tore into it when the AC compressor bit the dust, replaced the busted adapter and kaput compressor, and it's been blowing ice cold or lava hot as needed for the last 7yrs.