Re: dash goes crazy when I turn on headlights
while I was troubleshooting my dash lights 2 weeks ago I encountered all of the above when the center screw was removed from the dash cluster causing it to become ungrounded so that's my best guess.
Pull the steering wheel, unbolt the 3 nuts holding the column up and remove your instrument panel cluster.
I bet you'll find your cluster is not grounding well. You can bench test the lights on your dash by looking at them from the back of the cluster...
On the circuit board, top right where the connector goes there are 3 leads...the left most one powers the dash lights. Put 12v there and then connect ground to the metal housing that the high beam indicator snaps into. Does it work? If not, you've got a bad ground. In my case, the metal housing around the gauges was really scaley/rusty and I had to sand it all down. It then made a good ground and everything worked as expected.
2009 CTS-V - 1968 Camaro 327/M20 Sequoia Green