The mouse may appear in the car for several reasons:
The easiest method of keeping them away - use cotton balls, soaked in peppermint oil. Expand these balls in the car (if it is possible even under the hood) - and mice will disappear. Or purchase an ultrasonic repeller that works on batteries. This fully humanely and effectively. About one of the most popular electronic mice repellers, you can read in Battery Operated Ultrasonic Rat and Mouse Repeller Review.