1971 chevy,no tail lights and the back two side markers lights don't work either

[jtchevy355]

hello i have a 1971 chevy pickup,when i turn on my lights i have no tail lights, and the back two side marker lights don't work either,the license plate light works,my flashers all work except for the back two side marker light,and when i turn on the lights the tail lights and the back two side marker lights don't work either, i am lost i tried to find a ground somewhere but didn't find one,please help,thanks for your time