The mezuzah should be placed on the right of one going into the side room. Since none of the rooms are deemed to be internal to the other, one then looks at the main direction of traffic.
This is true even though the living room is considered the main room in relation to the side room.[1] This is also true even if the door is hinged so that it opens into the living room.[2]
[For general guidelines in determining the correct right side of any doorway, consult GENERAL PRINCIPLES, located in BASICS section on our home page.]
[1] Priority Three.
[2] Priority Four.