[LDO]

I have actually been looking into this. I believe the Silverado models (I have an 85) came wired for all the buzzers (headlights, key & seatbelts). The difference is in the buzzer module itself. You have to pull it and see if you have a 7 pin module or not. The 7 pin is the one that will buzz for key, belts AND lights. The other one (the one I have) has 4 pins and will only buzz for key and belts.

It is located over by the gas pedal.

I plan to go to a junk yard and look for a 7 pin one.

There are several GM PNs to look for:
22535998
22533568
are just two of them. These modules were used in other GM vehicles as well.

There are a few on EBAY but they want too much for them when I can pay a buck to get into the yard and the fella behind the counter will prolly just give it to me or ask for 3 bux.

None of the parts sites I've found carry them anymore.

Now you know everything I know about this.