[papajam]

The Giulia Spider wiring diagram I have also shows the lighter being powered with a 10ga red wire from the headlight switch (daisy-chained). This does not necessarily mean though that the wire, in the physical sense, jumps directly from the headlight switch to the lighter. It could go back into the harness and emerge in another location. The instrument light switch however is powered by fuse #4.
The lighter requires only two connections; power and ground. If you have a direct short circuit with the lighter hooked up, then, as Brent said, the problem is in the lighter.