Plants should not sit in an unheated car if outside temperatures are below 45 degrees. It is a good idea to cover plants when transferring from store to car on cold days.
Check the soil to see if it is dry on top before watering. Then apply enough water so a little runs out the drain holes. Plants are usually wrapped in foil. Do not let water accumulate inside the foil wrap. You may remove the wrap or make holes in it so the water drains into a saucer or the sink. Water in the foil can be reabsorbed by the soil so the soil remains too wet. Roots need oxygen as well as water. If all the soil pores are filled with water there will not be adequate air for the roots. When roots start to die, plants will lose leaves and flower bracts also.
Plants do best with plenty of light. Lamps near plants can be left on all night.