This is odd with the light on when the switch is off and key removed.
When I have done work that causes a regular "check eng." lite I just "erase" the codes by doing the following which I got from a mechanic: disconnect the negative lead on the battery, turn the ignition switch on all the way (not start), step on the brake pedal, and reconnect the ground cable. I have never had the light stay on or come back on so I assume that as long as the trouble is fixed the light will stay off. I don't know if this will help in your case.
It sounds like you may have a short or a cross some where if "erasing" doesn't help.