what is this electrical plug?
 

where does this go.it a lt green,dark green,yellow and brown

Rear Taillight stop, turn signal wires



http://www.geocities.com/chevroletc1070/taillghts.html

Try tail and stop light under dash plug on c30 up models.

it plugs into the firewall right above the gas pedal, and there is a harness that plugs into it and goes to all rear illumination

on all C-10 and up...not just C-30

isnt that the connector for rear lights at the rear crossmember on left side? light green is back-up, brown is tail lights, yellow is left turn-stop, dark green is right turn-stop.

it has the same plug on either end, however, in that picture it has heater resistor assembly, so it has to be the cab end of that harness.