[ray_mcavoy]

The 1978 & newer GM radios (like the one in Desert_Eagle02's photo) used connectors like the ones shown in the attached pic.

The blue plug (if used) is for the rear speakers. It's part of the rear speaker harness and will only be present in trucks that had rear speakers.

The white plug is for the front speakers. It's part of the front speaker harness. And for the mono radios with the center dash speaker, the front speaker harness was simply a short section of wiring that ran directly from the white plug (at the radio) to a plug into the speaker.

The black plug is for the power, ground, and dial light. This plug and it's associated wiring is part of the truck's cab / dash harness and should still be present unless someone has cut it off. The wire colors are yellow for switched power, black for ground, and grey for the dial light.

Finally, my pic also shows the black 2-wire connector (with brown & orange wires) that was only used in trucks with the optional digitally tuned radios.


EDIT, I believe the 2-wire plug with the pink/black & orange wires shown in your 1st post is for a TH400 kickdown switch that would be mounted to the bracket directly above the throttle pedal. The round plug with the black & white wires looks a bit like the ones used in 4x4 trucks for the 4 wheel drive indicator light (but seeing as your truck is a C20 it might be for some other purpose).