About 4 months ago, I installed tweeters and crossovers to the factory stereo system. Big improvement in sound over factory at very little cost.
I have a '01 SE with the 6 speakers and 6 disc in-dash changer.
The back tweeters were easy enough to put in....
I mounted the 2 crossovers on the sheet metal in the back of the trunk using double sided tape from Radio Shack.
To make the hole to run the speaker wire for the tweeter from inside the trunk to the rear deck, there's already 2 holes in the sheet metal nearby the speakers in the rear deck. The holes are about 1/4' in diameter. but you/re going to need to cut through the particle board of the rear deck. Use a drill and drill through those 2 holes.
You got the holes. You got the crossovers mounted. Time to do the wiring.
You're going to need a wire cutter, wire stripper, wire crimper, crimping set, and of course speaker wire
. Cut the factory wiring to the rear speakers. Connect up the power and the speakers to the crossover.
For the door speakers..........
Remove the front door panels. Remove the mirror panels. The tweeters are going to be mounted onto the mirror panels.
There's going to be a separate set of speaker wires going to the full range and the factory tweeter. The wires are connected to the full range and factory tweeter by a clip harness. Disconnect both harnesses and cut the harness of the wire going to the factory tweeter.
The factory tweeter doesn't do sh!t. And the signal coming from both wires, going to the tweeter and full range, are the same. There is no factory crossover to split the signal.
Mount the crossover again using doublesided tape. Mount it onto the inside of the door panel in the space where the speaker occupies. There's enough room for the crossover and full range when you put the door panel back on.
Connect the speaker wire that was for the factory tweeter to the crossover. Use the wire harness that was cut of for the tweeter, crimp that to the low signal from the crossover, and hook up the harness to the full range.
Mount the new tweeters to the mirror panel, either double sided tape or screwing it on. You need to drill a hole in the panel for the speaker wire from the tweeter. Connect the high signal to the new tweeters.
That's it in a nutshell. I didn't take pics the first time around because I was experimenting and I didn't know how it was going to come out. But I plan on doing this to another Altima soon, and make a detail instructions this time around.
I would suggest getting the Master Sheet from Crutchfield so you would have an idea on how to remove the front door panels and what it looks like behind the panel.