Here is a pic of the factory cab light connector. It seems like people ask about it from time to time so I snapped a pic. This connector is hot when the park lights or headlights are on. There are 2 wires in the connector that must be connected. If you cut the connector off, you will need to connect both wires to your cab light hot lead wire.

 

Why would you need to cut the connector off? Every 1980-1986 truck has that connector, if your truck has Cab Marker lights installed or not.

If you want to install cab marker lights, just get the lights and wiring, use a template to cut the holes in the roof, and plug the Cab Marker light wiring into that connector you have pictured.

There is another such connector under the hood for the marker lamp relay, or for camper or trailer wiring. Again not all trucks were installed with a marker lamp relay, but they all have the connector under the hood as well. One wire goes from the connector you have pictured to the connector under the hood. The other end goes toward the headlamp switch.

1980-1986 trucks a full of unused connectors for ease of installing accessories ordered and installed at the dealer, and for ease of installation at the factory.