Magic Trainer gives you unlimited ammo (all guns, grenade launcher and flamethrower) and police bribes. This version of the trainer was relevant for 1999, and most likely is the last one.

To run it, you must move the trainer exe application to the path GTA/GTAWIN/, and start the game. Otherwise, you will receive an error message. Once you enter the game, press ALT-TAB or ALT-ESC to return to the desktop. Now, after that, run the trainer program.

To activate the trainer, you can click on the "#" button. Alternatively, you can select activation keys from the drop-down menu, and you can also assign function activations to specific buttons.

Important:
The trainer only works with GTA for Windows, and with the old Retail version. Roughly speaking, there are 2 different versions - the version that was released in 1997, and the one that is a re-release of "RockStar Classics", released in the mid-2000s.
There is the easiest way to find out what version you have:This is one of the most notable features. You can get the Retail version, and even with pre-installed London GTA add-ons, you can here.