On an '02 S2K, is there already wiring in place for rear speakers behind the seats, or will it have to be run from the head end? Also, can 6.5 coax speakers be easily fit for rear speakers behind the seats? Are there recommended units that are a reasonable, balanced fit with the stock fronts in the <$200/pair range?

 

No current model year of S2000 has wiring run to the back. You must do this yourself. It's easier with the Lucid harness. Soldering those wires to the back of the HU is a pain. It's not the size of the speaker that will determine what will fit. It is the size of the magnet that has to clear the metal support posts back there.
Go to the Lucid site to see all the different combos you can have. He is a sponsor and you will find his website by clicking on "sponsors" at the bottom of the page.
You can get as fancy as you like and certainly $200.00 will get you what you want. I went the cheap route and put cheap Sony 1521's in the doors and used the stock speakers in the back. This might be of interest to you:
http://forums.s2ki.com/forums/showthread.p...%2520userid1612

 

I had a set of 6.5 kenwood excelon's as rears, but had to use a 1/2 inch spacer ring between the speaker an the rear panel. I wouldn't recommend anything over 5.25's if you want to retain full seat travel & recline...