Using DroidX Phone through Car System

[Seven3]

When I insert the wire from the DroidX into the auxilary input of the car speaker system the DroidX no longer captures my voice. With my previous phone, the original Droid by Motorola, I was able to have hands-free phone calls by plugging the Droid into the Aux input enabling me to hear the caller through my car radio speakers and the caller was able to hear me through the regular Droid mic. With the DroidX I can hear the caller through the car speakers, but the DroidX mic is not picking up my voice so the caller can not hear me. Any help is appreciated. Thank you. Jeff