I have just (last weekend) fitted 2 amps, a sub and 6x9's in my 172 off the standard head unit as i to wanted to keep the steering wheel control stick and didnt really want to change anything as i like my 172 the way it is
The speaker wire to rca adaptor (dont know the technical name) is the correct way to go but depending on the sound qulity that you want depends how much you spend on the adaptor and which one you buy i ran the + wire from the battery and heres the CLEVER part..... where can you run the remote (a 2nd + to turn the amps on) wire from? i was going to run it from the ignition wire on the back of the head unit but decided against this as the amps would be on if the key was in the ignition, if u have a disc changer you can run the remote for your amp from the power for the disc changer as this comes from the head unit and turns on with the headunit, its the grey cable from the desc changer plug (in the back of the head unit) just get a mutli meter out and check the volts when off and on u should only get a reading of 11.5 - 12.5 when its on and thats with the car not started, as for the 6x9's DO NOT cut up your parcel shelf they will fit either side of it in the standard speaker spaces in the boot (there are no speakers there)
now it is a tight fit and you dont want to screw them in :nono: you need to twist them in (i know what ur thinking lol) you push in the 6inch part and twist to get the 9 inch part in then there are 2 holes in the plasic just put 2 cable ties through the screw holes in the 6x9's and the 2 holes in the plastic and as you have twisted them in they are solid the cable ties are just to stop them untwisting
run the speaker wire from the rear speakers in the door cards (the + is the one closest to the clip for the adaptor, on the passenger side its a grey cable and on the drivers side its not the grey one as thats the -)
And the best bit about all this is you can take it all out and noone would ever have know it was there and if some one looks in the car there are no 6x9's visable in the back parcel shelf and you have not cut up and filled your car with screws
One last thing getting the cable through the engine bay is a right pain look at this
http://www.cliosport.net/forum/showthread.php?t=143341&highlight=Reservoir
Sorry if thats a long post but it needs explaining correctly... I hope this helps mate, any questions PM me... happy to help.
Mark