To find the Heart of the City you must do this:

1. Type 359-555-0100 on your cell phone or steal a helicopter.
2. Fly to Happiness Island and jump from the heli to the highest balcony of the statue.
3. There will be 4 doors there and one of them will be possible to enter.
4. When inside, grab the ladder with (Y) and go to the top.
5. You will find a litle surprise there but I won't tell what is it or the fun will be lost.