[Showdown]

Plywood will hold moisture just as well as concrete, in fact the wood will be quicker to absorb ambient moisture, though it will also lose that moisture faster.

You could lay down a rubber roofing membrane if moisture is a concern but that would trap the moisture in the concrete and that will shorten the life of the concrete- not radically but still.

The best thing would be a matrix that allows airflow laid down on the concrete and then a moisture barrier atop that.

The simplest thing may just be to install some sort of air circulation under the car so that moist air is removed from that space. An array of computer fans would do the trick. Just a bit of airflow makes a huge difference.