If the stereo in 2002 Navi has no bass and all trebile what is the deal speakers are fine and don't crackle just zero bass?
It may very well be the subwoofer in the back of the truck passenger side panel, could be the amp or the subwoofer is toast. I had to replace one on my 98 gator, but it was noticeably blown. It requires taking off the entire rear side panel. The subwoofer is bolted on through the backside of the bass box so the only way to get to it is a full tearout. hope this points you in a direction. There are other articles on here about replacing the woofer&.
there is no fuse for your speakers the speaker wire runs from your stereo you might have a wire lose behind your stereo or may have shorted out some were from the stereo to the speaker or some times you might put some thing in the trunk and disconnect the wire from the speaker and if that just plug right back in to the speaker