My P 30 van's flashers, dash panel, tail and brake lights do not work.

My P 30 van's flashers, dash panel, tail and brake lights do not work. My lights,tail, brake, dash and flashers do not work. When I turn the lights on sometimes for about 30 seconds they come on but always go out. Sometimes I'll have brake lights but they never stay on much longer. I've changed all the fuses twice now. I've been trying to run shorts down but I am now at a wits end.I have also changed the light switch and ingnition switch. Help. I have a very limited income and I live in my chevy