When I transport chicks, I crank my heater all the way up, and dress for summer weather, even if I'm transporting chicks in a February snow blizzard. Yes, I've been there, and done that. That means the chicks may be more comfortable than me. An other option for you would be to use a power converter plugged into a cigarette lighter, and use a small heating pad wrapped around one end of the box they are in. The bigger concern IMO would be that they may not have had a chance to fill up on feed and water before you pick them up. Chicks arrive from the hatchery being totally depleted on their nutrient reserves. They need a few hours to stabilize, rehydrate, and get their crops filled up. Allowing that time would help ensure that they not get overly stressed by their SECOND journey. While you are at it, pick up a bottle of Poultry Nutri Drench to help them get off to a good start.