Found a workaround:
I downloaded the RPGM2003 RTP from the official website, here: https://www.rpgmakerweb.com/run-time-package
Then I opened the downloaded Installer.exe with 7z.zip as an archive.
Navigated to rpg2003_rtp_installer.exe\CharSet
Copied the file "Vehicles.png" to [Professor Frippel's game directory]\CharSet
And the game works now. There's a "vehicle.png" in the game's CharSet folder, but capitalizing it doesn't seem to fix anything. I had to add the new one to get it to work.