My 2016 Ram did not come with a Cabin Air Filter, it did not even have the Door to access it.
My Ram was built in December 2015 !!
The Rams built
in 2016, may have the Door already installed, you just had to remove the door to install a Filter
I bought a kit from the Dodge Dealership & they cut the HVAC box for it to fit & they installed it.
Later i found out, that GENO's Garage sells the kit
Add this Dodge Ram cabin air filter prior to the HVAC blower motor box to keep the honeycomb grill that covers it free of blockage caused by excess dust. Installing this filter kit is an easy DIY project that costs less than $30.
www.genosgarage.com
Then there are several YouTube videos on how to cut the HVAC unit, to fit the door
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now the kits are available from several sources