This post is meant to be informational and a discussion for people looking for info on the audio possibilities of the SX4.
The rear door panels are easily removed by removing a screw near the door handle and one in the arm rest. Pull the door panel off starting on the inside door jamb. The panel seems to over hang the door jamb making it easy to get a handle. Unclip the power window plug using a small flat head screw driver. Either 5.25 speakers or 6.5" will fit. Some 6.5's are too big however so make sure they will fit before purchase. Suzuki was nice enough to put a spot for a tweeter in each door panel so you can put two pair of component speakers in the SX4. Those speakers that will not fit might be adaptable using a 1" spacer ring available on ebay.
Quick edit. Be carefull of tweeter size. I have both front and rear door panels off now and both of my tweeters are a might too big. thus I must decide whether I want to modify the panel or buy new tweeters. Ugh!
Midrange size has become an issue as well. The front door does not have much room for a magnet. You can use a 1" spacer for 6.5" speakers. The Metra part No. is 82-4300.
Additionally, while looking on crutchfield, I found that Blaupunkt offers a wiring harness to add an amplifier to a stock system. I am not sure if the harness works with the SX4, however, crutchfield says that it does.
Link: Vehicle-specific Amp Cables at Crutchfield.com
Note that there is plenty of room behind the factory stereo for additional wiring. No antenna adapter is required for the SX4. The following wiring harness and installation kit are reported to work.
Dash kit Metra part no. 99-7954
Wiring harness Metra part no. 70-1721