I have an 03 chevy venture the dash lights don't work and when I turn on the headlight switch the radio lights dem and go off why is this

Check/replace the ignition switch for wear, pitting, or melting.

Intermittent electrical problems on GM vehicles (http://www.ricksfreeautorepairadvice.com/diagnostics/by-symptom/intermittent-problems-on-gm-vehicles)