This can get very expensive!! What I did didn't cost a lot but it works very well.. Dont get too hung up on wattage figures, as long as the total speaker wattage is higher than the amp output. I'm running 2800 watts worth of speakers from a 400watt amp, and it will blow your head off! It's all about the ohm rating.
Buy a decent power amp (not a chinese cheapy from ebay) I got a JBL 2 channel stereo amp and stuck it in the glovebox.
Wire an ignition live from a switch and wire the speakers in parallel. Most amps run at 4 ohms a side, so make sure both speakers are 8ohms then wire like this,
The ones in the diagram are 4 ohms resulting in 2 ohms. Only expensive amps will run at 2 ohms.
If you have 2, 4 ohm speakers you can wire in series which will flip the ohms the other way and not over load your amp
Never overload the amp, it will not last long. The lower the ohm calculation the more draw there will be.
Full range speakers dont need crossovers, only needed when running tweeters and bass bins.. leave that to the saxo drivers..!
I just wired a 3.5mm jack to the amp so I can listen to my ipod, not a big fan of the radio but I can plug my phone in and listen to the radio that way. You can use the speaker out from your original radio (unless you have a modern radio with an unpowered signal out aux, then it will be ok) and wire to the amp, but the amps that can handle a powered signal input are expensive.
Have you ever used one of those fm transmitters before? they're shit.
Nothing beats a hard wired system...
I wanted to leave the orginal radio as is and run the new set up separately. My orginal radio only had one speaker on the dash top anyways.
A few pics
Make sure all speakers are wired properly, if not they will run out of phase resulting in the bottom end cancelling out....
Good luck!