Carriages and buggies had a "dash" board that were made of drop cloth or leather. The board kept the horses feet from slinging up mud or poop as they dashed along through puddles and muck keeping the driver and passengers clean.
When motorized vehicles were introduced, the dash board adapted and was used as the instrument panel. Trunks were derived from carriages and buggies, too. At the back of them, trunks, or suitcases, were strapped on for traveling.
Therefore, when the motorized vehicles came around, they added a back area to store things and kept the name trunk, since it is what had been used previously. It was very logical at the time. Several parts of the car are named after practical parts of the carriage and buggy system.