I'm assuming your frames are numbered 1 to 8, with the car rotating clockwise at 45 degree increments, starting with it facing directly upwards.

Try this code, in the car's object actions:

Code:
diffx = _x - _root._xmouse;
diffy = _y - _root._ymouse;
a=((Math.atan (diffy/diffx)) * (180/Math.PI)) + 90;
if (diffx >= 0) {
  a+=180;
}
frameNum = math.floor(((math.floor(a/22.5) + 1)%16)/2)+1;
this.gotoAndStop(frameNum);
There might well be a simpler way to do this, but I'm relying on my feeble mathematical abilities...