Running both CB leads from the battery is an excellent suggestion - it helps because it keeps any ignition noise in "common mode" (i.e. induced equally in the ground and power lines, which cancels out) which reduces its impact greatly. It also results in much less voltage drop since your CB is the only load on the circuit and goes directly to the battery instead of sharing with the blower motor, ignition, headlights etc. Use heavy gage cable - I used 10ga boat cable for my ham radio, though its power draw is higher as it produces 65 watts of RF rather than the 6-12 permitted from a CB. Another thing you can do if you still have noise issues after upgrading the power wiring is to buy a few large ferrite rods (some ukrainian guy on ebay is selling packs of 10 fairly large ones for a very reasonable price), tape a few of them into a bundle, and wrap your power cable around it about a dozen times to form a choke. If you still have RFI issues after that you can add a few 0.1uF ceramic capacitors across the power and ground wires before and after the choke to form a pi filter.
EDIT: also make sure you have a good ground at the antenna - don't just clamp it to sheetmetal or something carelessly, run a piece of heavy gage wire or braid from the coax braid to a bolt in the frame or a body panel.
