First, find one of those green hexagonal gates and enter it (The easiest one to do this on is the one leading to the Industrial Zone level).

Play through the stage until you find the large "EXIT" sign. Crash into one of the letters (It doesn't matter which one, but the easiest one to hit is the E). Usually if you crash into something you will die, but you will still be able to complete the stage even if you crash into the EXIT sign, but nothing else.

When you exit out of the gate, a 9X Multiplier will appear out of the gate. Grab it and re-enter the gate, and do the same trick. This can be done until your Multiplier level is at 99.

It CAN increase, though, by destroying Multiplier carriers (the jet you can control and the van that tries to run over you). The Multiplier value will glitch after a few Multipliers, but it WILL be above 100, and can still increase.

Note: This code will NOT work in CyberWar Levels or one-way gates (such as the Harley's Gas-Er-Up warp gate leading to Cyber Run, or when you are at the stage where you can defeat Jobe).