Norm,
Wiring will affect the compass if you are running large amounts of current through it and it is close to the compass. Pretty well all electronic equipment stipulates a compass safe distance of 1 meter, or 3 feet. Twist the wires and keep them away from the compass. In reality It does not really matter if you compass is affected, i.e. has an error, as long as it is constant and you know the error. It is the intermittent error, such as the lights on at night that induce an unknown error, that might give you a head ache, as you may not be aware of it.
without the aid of a gyrocompass it will not be easy to determine the error of your compass. Use range lights and markers and known headings to determine the error.
Michael