I like a lot of airflow and a minimal interior. My car has several vents and holes in it, from deleted trim strips, places that wires pass through the firewall, drains, etc. This is fine for driving, even when it's wet. However, when the car is parked during a rainstorm, afterwards water condenses on the ceiling and inside the windshield and rear window. It takes several minutes of driving to heat up the glass again. What can I do to prevent this?
First, you have to reduce the amount of water coming into the car. Sealing up those holes is a good place to start. Holes in the firewall must have a rubber grommet in them to prevent the wires from rubbing against the metal of the firewall, potentially causing a short circuit.
Once you've done that it gets a lot easier to deal with the remaining condensation. Wiping the inside surfaces of the windshield and other windows with an absorbent cloth should be enough when you start driving.
You can use a dehumidifier (a chemical absorbent) to keep the car dry.