Hi Jaseel,
Meet your new friend
Mice can come as a surprise. I suggest you have the entire car (under the hood, dash, trunk and seats) cleaned and checked, then you can spread the mice poison for a day and leave it there. Have the inside of the car cleaned again - so that you don't inhale any poison while driving and see if they return.
And good idea about spreading the mice poison every time you leave the car for longer periods of time!