Let me clear up the speaker spec/size info posted here since some of the replies are incorrect (at least for a '02 IS300). All info is from my 2002 IS300 sedan and pertaining to the rear deck speakers.
Stock speaker: (2) total speakers per location (ex: rear right speaker, rear left speaker)
A
woofer (bottom--in a metal housing) and a
tweeter (top--mounted above woofer in speaker assembly but in its own housing).
Both
woofer and
tweeter are wired independently from each other but meet at a 4-pin connecter that connects to the wiring coming from the factory amp in the front of the vehicle.
Speaker specs:
Size -
Woofer: 6" x 9"
Tweeter: 2 3/8" diameter (or 2 1/4" depending how you measure)
Technical Data -
Woofer:
Impedance - 2 Ohms
Power Handling - 25W
Tweeter:
Impedance - 4 Ohms
Power Handling - 17.5W
My opinion:
I personally wouldn't replace the rears with "simple swap" options for a few reasons, namely price. Aftermarkets would be much cheaper (depending on brand) but will require you to change the current wiring a bit based on your install. The wiring upgrades for the most part should be under $35 and pretty simple to install as long as you don't hop on the forums and get advice from 10 different people. A couple possibilities are wiring them directly to your aftermarket deck if it's powerful enough or installing an amp and running them from that (again, additional wiring changes necessary). There are a lot of options these days for 6x9's including extremely efficient models that output a ton of sound with very little power demand. Something else to think about is the fact those speakers in your particular car don't need to be the loudest or best quality. They are considered to be SECONDARY to the door mounted 6.5" component speakers and only really there to supplement the primary speakers (doors). True, they balance the sound field out nicely but unless you're sitting in the back seat or have the radio faded to the rear the sound you're getting while driving is from the doors.
P.S. If you go the aftermarket 6x9 route you will need a speaker adapter mount in order to install the new speakers in the rear deck properly. The ones I bought are simple, thin and allow me to install the speaker into the mount and then to the rear deck using the factory holes. No drilling or additional screws required! I attached some pics to show you what I mean.
Stock speaker vs. aftermarket 6x9
Stock speaker vs. aftermarket 6x9 (with speaker adapter attached)
www.crutchfield.com
www.facebook.com
