Voltage drop when I turn the key or lights on

I have a 2003 Chevy Tahoe. I recently changed the starter in it and everything worked fine for one day. It was starting better than it had ever been and I started it four or five times then went to town. All was great. This morning when I went out to get in it I turned the key and the only thing that would happen is that the check engine light came on. It acted like an electrical problem the other lights flickered on but then went off. I did the obvious and took the starter back off and checked all the wires and put it back on still the same thing. I got my meter out and at the battery it showed 12.65 volts but then I turn the key on and it dropped to 5.46 volts. it does the same thing when I try to put the headlights on. I've checked every fuse I know and checked all the grounds, cleaned them up. Also, the Dinger does not go off when I turn the lights on or put the key in the ignition either. I've chased wires all day and I'm at my end. Can it be the ignition switch.