I have a 2005 Express 3500 6liter. When the ignition is off, the instruments light up. When it's running, they still light up, plus... ABS light is on, no horn, no cruise control. It starts up and drives OK otherwise. Fuses check out OK. Got any ideas?

 

I would first try pulling the cover off of the PCM and checking to make sure that you don't have any exposed wires that could possibly short on another wire or metal. Could also possibly be some wires from the ignition shorting out somewhere along the line.
The last option I can think of right now is to check all of your grounds to the chasis because when not properly seated they can cause problems... I have also had a burnt out taillight make my dashlights come on when I would push the brake pedal. Bad grounds can cause some funky "unexplainable" issues.

 

+1 on the bad ground idea

If a circuit can't ground through its intended ground point, it will feed back through other circuits till it finds a ground