I recently changed out the speakers (front and rear) on my 1998 Altima and while I was eventually able to do it, it took quite a while to figure it all out. To replace the front speakers, you'll need to pry off the door panels (at least enough so that you can get a screw driver in there). I can't remember exactly what else I removed but be prepared to remove the housing around the door handle, window switches, etc. The back was MUCH more complicated as the speakers are only accessable from inside the car and this requires you to remove multiple interior panels and molding so that you can get under the rear deck. It is confusing at first but once you start the process it becomes clearer.
Try to use the Nissan speaker adapters plugs so you don't need to cut wires and mess up the electric system, you can get those from eBay or amazon.
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This should answer your question? I used it to help me replace mine on my altima. forums.nicoclub.com/zerothread/157923
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1998 Nissan Altima GXE has a timing chain. Check your model at: http://www.courtesyparts.com/watches-c-7738.html
when it breaks More info here: http://www.nissanforums.com/l30-1998-2001-altima/125696-timing-belt-chain.html