p_vehicle might never be exactly at position special_entity.x.
Perhaps, you should use a distance check instead.
Code:

---

define _quitTriggerDist, skill1;		//set this in WED
//uses: _quitTriggerDist
action special_entity {
	while (p_vehicle == null) {wait (1);} // wait until the player is created
	while (my != null) {
		if (vec_dist(p_vehicle.x, my.x) <= my._quitTriggerDist) { // the player has come close to the door
			quit();
		}
		wait (1);
	}   
} 

---

I probably wouldn't use the door entity's position. I would probably use an additional entity which might placed inside of the garage.