The coolant reservoir on a 2001 Ford Mustang (like most vehicles) has
high and
low marks, not "full cool" or "full hot." You should
always fill the coolant reservoir to the
cold fill line (the higher mark). This accounts for expansion as the coolant heats up. Filling it to the top when cold will cause it to overflow when the engine warms up.
You'll find the coolant reservoir bottle usually in a readily accessible area under the hood. It's often translucent or semi-translucent plastic and clearly marked with the high (cold) and low fill lines. Look for a tank labeled "Coolant," "Antifreeze," or something similar.
