MrPaco,
On my ’88 the buzzer is located just above the hood release handle.
PICT0021.jpg Two wires, a yellow w/white stripe which is from the door switches, and a green w/orange stripe from the ignition switch, connect to the buzzer. Hopefully your buzzer is there and only disconnected. If there and connected, check fuses #1 and #7 and the buzzer itself. The buzzer serves as both a key and fasten seat belt reminder.

To wire the buzzer as a headlight reminder, you’ll need female blade connectors, some 14 gauge wire, a quick splice connector and a crimping tool. A volt meter or voltage tester is recommended. Disconnect the green/orange wire and prevent it from grounding. Attach only the yellow w/white stripe wire to its original buzzer terminal. Use the original connector or crimp on a new female blade connector. Attach an 8-12” length of new wire to the other buzzer terminal.

Find the harness connector with a green w/white strip wire that powers the dash lights. On my truck which has four stock speakers, this connector is white with yellow side rails and 10 wires in two rows of five.
PICT0026.jpg On trucks with two speakers, this connector has six wires, one row of four and the other with two at opposite ends of the row. I don’t know its color. Hopefully it’s white and yellow, too.

On either connector the green/white wire is in a corner location. Confirm it’s the dash light’s wire by back probing the connector using the volt meter or volt tester and with the headlight switch in the on and off positions. If correct, touch the end of the wire you connected to the buzzer to the volt meter/tester lead with the lights on and off. If it buzzes when on, splice the new wire to the green/white wire to make the change permanent.
PICT0023.jpg
The pics show my test-of-concept using a wire with alligator clip ends and a male pin connector to back probe and connect the 10-wire connector’s green/white wire. I just never got around to making it more permanent. You can also connect the new wire to the load side of fuses #1 or #12, but the connector I used is more accessible.

Hopefully the above matches your ’87. Also, with this change the buzzer will no longer sound as an unbuckled seat belt warning. However, the fasten seat belt indicator light in the dash cluster will illuminate for 5-10 second like before.

I hope this is clear.