label each of the 8 frames as thus

left
right
up
down
right-up
right-down
left-up
left-down

and then on the car movieclip, assign the following actionscript:

Code:
onClipEvent (enterFrame) {
    mousex = _root._xmouse;
    mousey = _root._ymouse;
    currentx = getProperty (this, _x);
    currenty = getProperty (this, _y);
    if ((mousex > currentx )and(mousey > currenty)) {
        gotoAndStop ("down-right");
    }
    if ((mousex > currentx )and(mousey < currenty)) {
        gotoAndStop ("up-right");
    }
    if ((mousex < currentx )and(mousey > currenty)) {
        gotoAndStop ("down-left");
    }
    if ((mousex < currentx )and(mousey < currenty)) {
        gotoAndStop ("up-left");
    }
    if ((mousex = currentx )and(mousey < currenty)) {
        gotoAndStop ("up");
    }
    if ((mousex = currentx )and(mousey > currenty)) {
        gotoAndStop ("down");
    }
    if ((mousex > currentx )and(mousey = currenty)) {
        gotoAndStop ("right");
    }
    if ((mousex = currentx )and(mousey > currenty)) {
        gotoAndStop ("left");
    }
}
although, it would look better if you used trig to smoothly rotate the vehicle as stickman said.

this'll do as a substitute if maths and physics scare the living beejesus out of you.

Bruce