I traded an old trolling motor for a Garmin 140 fish finder. I noticed that the back light stays on even when I turn unit off. Manual says I can toggle power button to change light from off, low and high. Light just stays on as long as plugged to 12v. Anyone have an idea of whats wrong? How much more batt drain will I have with light on? I remove from boat when not in use so no probs storing.