Well, I don't have schematics on the 99'*, but may be able to offer a little help. When you checked for power at the fog light connectors, did you use an independent point on the chasis (one probe of meter on +12v pin, other probe on steel frame of car) , or did you place meter across the two pins in the connector? It could be just a simple open in that ground ckt. for the two fog lamps. But since you said the fog light indicator LED is not illuminated either (with ign on & press fog light sw), could suggest another prob in a different area of the fog light control ckt. In the 2000+ Bonnevilles, the Dash Integration Module (DIM) provides ground(when request for fog lamps initiated) for the fog lamp relay. The DIM, and the Instrument Panel Integration Module (IPM) are linked via serial data bus. The fog light switch is direct wired to the IPM which monitors the state of the fog light sw "open or closed". The 99 Bonnevilles may be similar in operation. It can get somewhat complex here. Do your high beams operate normal? They are interlocked with the fog lights. I'm sure some of the more knowledgeable 96-99 guys will probably jump in shortly. Good luck with it.
2003 SSEi, Light Bronzemist Metallic - All Stock For Now
IBEW Local 261 Groton, CT
My other vehicle is a nuclear submarine.