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There is a small fuse box under the hood.... not the big one.... there is a smaller one. It only holds about 6 fuses. There is an audio fuse in there. It is on the drivers side, close to the brake booster.
That little fuse box was only on the 97 & 98 models and only contained two fuses: One for the PCM KAM and the other for the trailer stop/turn signals. By the 99 model year, it was eliminated. It never had any fuses in it for the audio system.
There are two fuses under the dash that the factory radio uses. Check both of them. Verify that they are actually passing power to the connector for the aftermarket radio. Check the aftermarket radio's harness for any more fuses, some manufacturers add their own.
It never had any fuses in it for the audio system.
The hell it didn't, unless it is just an Expedition thing. My Expy has all but one fuse in that block.... and 2 are for audio - one is for the amp, and one is for the radio - I know because they were both popped when I got it and nothing worked.