Mine is stereo with either headphones or the cassette adapter (remember cassette tapes?) that I use in my work truck. If it's not going all the way in it won't be stereo. Try another plug in acc. to see if it is just your adapter.

 

It's because your plug is not going in all the way. I have experienced the same issue with my headphones that do not clear the rim of the housing, the area where the orange seal meets with the phone to provide water-proofing/resistance. For my car adapters I have shaved off a bit of the plastic (I used a Dremel but you can use a razor blade too if you don't mind the cut marks) surrounding the metal plug so the entire plug will go in.

My Sony MDR-V6 headphones have a large metal plug so I don't use those. Thought of making a custom adapter from an old pair of earbuds that fit...but we'll see.

 

i had an old Y splitter from radio shack i cut down, and keep in the car, im pretty sure it was like 99 cents when i bought it.

i bring this with me to places i doubt will have a modern connector, which is most anywhere still it seems llol