8Racing::Jon::RMS13
sigh...
Installing 6.5" speakers in the front of an S13

This has been done by many owners, because as you'll see, the door opening allows for a much larger speaker than Nissan supplies, even if you have the Active Speaker package. I've never heard the stock system, as I inherited an aftermarket deck, amp and the 4"x6" speakers all around that you can see below.

I used masonite (3/8" - I forget), and actually did a pretty sloppy job. If you wanted to make a more serious effort, you can use heavier material, seal the hole more completely, and use dynamat. It gets expensive quickly. Nevertheless, I noticed a big difference. Here's how it went...

 
 
As you can see, the hole is quite large. The 4x6's came in a small box with a back, preventing them from developing any bass. Hmmm...
I had these speakers left over from my last car, so might as well use em. Pretty decent Panasonic units.
First, I made a tracing of the small box that the 4x6 came in onto a cardboard template. Use this to find out where you need to drill holes to mount the actual "bracket" to the door, and where to drill holes to mount the speaker. Don't forget to make sure your speaker isn't too deep that it doesn't clear the window.
Next, I traced it onto the masonite and used a jigsaw to cut out the form...
...and marked where I needed to drill.
Drilling the holes for mounting to the door.
I used a drill to make a starter point for the jigsaw...
... and made the speaker cutout. Careful not to let the jigsaw run away from you, or get stuck as it could crack the masonite or cut outside your circular tracing. Also avoid twisting the blade, as jigsaw blades are thin, get pretty hot and like to break. No, I did not break any blades (thank you very much), but I did ruin one of my pieces and had to start over. Good thing I had a lot of masonite to work with.
By now you should have something like this. Make sure the speaker fits, and that its holes still line up, and the the bracket's holes line up with the ones on the door.
As you can see, the hole is quite large. The 4x6's came in a small box with a back, preventing them from developing any bass. Hmmm...
One last check to make sure it fits on the door. If you're off a little bit, when you start putting the screws in, it will stretch and your piece may not fit at all.
Fin.