1991 Chevy Corsica electrical problem

Electrical problem
1991 Chevy Corsica 4 cyl

When I start my corsica and turn the headlights on they will not light and the dash lights go out and the radio loses power. My power door locks will not work and the dome light will not work. There are a couple of times when everything works fine but most often not. When I start the car and the radio comes on, if I open the drivers door, the radio goes off, the headlights will go out and the dash lights will not light. The blower motor runs fine.