Note that activating this cheat disables achievements on the Xbox 360 version, and trophies on the PS3 version. So do not save your game after triggering this cheat if you have any hope of unlocking achievements/trophies on that save file in the future.
First, steal a Mr. Tasty ice cream truck; one can usually be found driving around the bridge in Middle Park, Algonquin. While in the truck, click the Left Thumbstick to trigger the music. You can cycle through the different tracks by clicking the Left Thumbstick once to turn the music off, and again to start the next tune. One of the tunes is unmistakably the Grand Theft Auto IV theme song. When you hear this tune, call ZiT! at 948-555-0100, and then wait to receive a text message from the service. Once the text message has been received, read it to restore Niko's health and armour to full. This video is from our unofficial game guide. You can view the full guide here