99 Park Avenue Ultra, radio and windows failed at the same time

[Pegleg]

The radio, power windows, and the buttons on the steering wheel that control the heater and radio have failed at the same time on my '99 Park Avenue Ultra. We have swapped out possible relays, circuit breakers and fuses; we've even tried changing the BCM and keyless entry module (as suggested on another site) with no results. After checking the fuses under the back seat, the 10 amp fuse to the amplifier has power but the 5 amp fuse to radio does not, after jumping it with a live wire, only the radio powered up. Somewhere in the car these three items come together and we are completely stumped as to where. I would be VERY grateful for any direction you could provide.

Regards,
Pegleg

 

[Drakito]

Check the main power box under the hood. There are several large maxi, maxiplug, and other style fuses under there. Also check the ignition wiring and make sure your ignition switch has not gone out. GM ignitions sometimes stop working on one or more contacts internally. A multilmeter and a few minutes will let you know if it is still working.
If it has Class II databus architecture make sure nothing is shorting out the diagnostic plug in the kick panel