Usually put it in a box or cake carrier. That keeps foreign debris from landing on your cake.
Then experience and judgement will help you with where to place it. I like a rolled up towel to fill when the seat cusion and back meet in the front seat, then another towel on top, layed flat, to keep the cake from slipping.
You can lay it flat on the floor in the passengers side, again with a towel to keep it from slipping but there is always the chance of a steep incline or hill. The cake can slip inside of the box and get ruined, so I usually like the passenger seat in case of the short stop