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Sounds like you both need an owners manual for your particular vehicle. In it you will find all the fuses and relays listed.
I am more familiar with the '07, but if you download the manual for the '04 you should also find the info. It won't be the same for the two models.
You have relays for the trailer lamps. For example there is a left turn relay and a right turn relay. On the '07, these will be found in a second fuse box under the hood. Each circuit will have a fuse also. Is it possible you only checked the fuses under the steering column in the cab? Look for the others under the hood.