Flat, flat, flat surface!! No back/front seat!!!! Use non-skid material that can be found with the shelf liner under each of the cake boxes. Make sure you put some icing between the cake and cake board to prevent sliding on the board. Drive carefully, watch for bumps+ and sharp turns.
Welcome to CC!!
definitely use the non-skid stuff. it is awesome!!
If you can, do not place them where the sun will be directly on them. Even if the cake is ICED to the board, if it is warm enough, it will slide and your icing will melt or get a faded spot.
I always put it in a Rubber Maid plastic bin that looks like an organizer.
w/ that non skid rubber (new and clean) thing you use for under the rug.
No sudden stops or sharp turns. I drive my husband crazy when he is driving and i'm in the back yelling and screaming. 
Something I always do too, since it has been so HOT HOT HOT everywhere, especially here in Texas, I always start my car a few minutes before loading cakes and supplies into it so it has time to cool down. I drive an SUV and load my cakes into the very back cargo space, which is the flatest part in my car, (on the grippy-no-slippy shelf liner that everyone here is talking about of course). So I start the car a few minutes earlier so the A/C can have time to reach the back of the car too.
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