This repo contains code to build an ASI plugin that enables radio stations in all vehicles in GTA III. You can build it from source or download it from the Releases tab.
Place the gta-iii-radio-for-all-vehicles.asi file inside mss directory in your GTA III install directory.
This code has been tested with the following English versions of the game: 1.0, 1.1, 1.1 Steam, 1.1 Rockstar Games Launcher.
Thanks to ThirteenAG for the basic idea for the plugin (https://www.gtagarage.com/mods/show.php?id=23900). His version also allowed all vehicles to use Pay 'n' Spray which changes the gameplay. It also lacked the support for Steam version of the game.
Also huge thanks to aquaz, halamix2 and pitpo for the support during the reversing & debugging session. I described the whole journey in a blog post.