Definately sounds like a wiring problem. For the simple fact of:when the car turns, problem happens. Its the inertia in the turn that moves things. Try checking your wiring to both speakers on left side, make sure they are tight. If both are good, check speaker wiring on the back of the unit, and harness. If they are both good, check your balance on the stereo settings and make sure its equal L & R.
**Working PART-TIME for Carjunky for a while, Please bare with me =)**