Definately due to your proximity to the speaker, the right side is more aimed towards your right ear than the left being aimed away from you, some cars will be worse than others, particularly the closwer the speaker is to the floor and where your leg sits. You can calibrate your car stereo by taking a mono track of one guitar and move it around until your guitar is balanced but remember that anyone in the passengers seat will be unbalanced even worse.
We tend to critique out mixes worse than others, typically incar stereos. Listen to other mixes and pay attention to the balance. Most of use tend to not listen to mixes when we drive, but when its our own mxies we are listening for eq and volume balances andnotice little flaw in the mix and in the car stereo setup. The acoustics and speaker placement in cars are not the most ideal.