I'll assume you're talking about a 12V/1.5A secondary transformer, in which case most likely the primary will be 120V, and you use an inverter that steps up from 12V to 120V to provide the AC primary voltage.
The other way is to determine the power supply circuit (if it is 12V/1.5A), and jump it or remove it and solder a cigarette plug on. That's the easier method, although you'll need to have knowledge (or post pictures so I and the other forum guys can help) to bypass the rectifier circuits.
Amplifiers with inputs of 12V are usually designed for car use and are simply wired into the electrical system. This isn't relevant to the other parts of the answer, but this is an 18W max design - it begs the question - why do you want to use it in a car? Even most low-end head units can at least go 50-100 across all channels.
