Ususlly, electronics that come with a 12V wall wart adapter can actually take up to 24 volts. I took a cheap-o Wal Mart $20.00 DVD player and opened it up. I bypassed the voltage transformer (after verifying that the transformer actually reduced power to 12V) and replaced the AC cord with a panel mount barrel connector so I can plug it in to a lighter socket or power it with a 12V wall wart in the house.
Using an inverter is a really inefficient way to power low voltage devices. You are taking 12V DC to 120V AC, and then plugging a device into it that in turn reduces it back down to 12V DC. If you are handy with a soldering iron and a multimeter, you can do this.