You'll always get noise from a cigarette lighter type outlet. Going direct to the battery will help a great deal with ignition noise, but it will likely not eliminate it. You need to determine if the noise is coming in through the power line, or the antenna. Disconnect the antenna and see if the noise is still present. Does the noise change pitch with engine speed?
Keep your antenna cable as short as possible.
Keep your power and speaker leads as short as possible.
Install ferrite chokes (Snap-Together Toroid Choke Core-2-Pack - RadioShack.com
) in both power and speaker lines as close to the radio and speaker as you can.
If it's REALLY bad, you can start installing flat, braided grounding straps between every body panel to bond everything together electrically.
I also found that a lot of my TJ's radiated ignition noise was actually coming from the exhaust system, which was acting like an antenna. Running a couple of flexible ground straps between the pipe and the frame helped tremendously.