Ok it works! Yahoo!
But...(not again

)..
since this is my 1st time to use trace, i dont know what vectors to input so it traces from its front.
I put:
action car
{
while(1)
{
vec_set(myfront,my.pos);
myfront.x += 10;
trace(my.pos,myfront);
etc...........................
I know x is front. But when i run, it doesn't change when the car is turning.
it uses the world position 'x', not local position 'x'. Oh no! what do i do?
Forgive me for being a bad programmer.