Here is a guide taken from saxperience.com, written by infinitym4ge
A saxo is actually a very easy car to fir wiring, due to their poor build quality. It is easy to reach from the foot well straight to the back of the radio!
Personally I wouldn’t remove the seats, unless you are using very thick cables.
There are many different ways of doing it, a lot of people will say you don’t need to remove the front seats or unbolt anything but honestly that’s just the bodged way of doing it.
Hopefully by reading this guide you will get the idea on how to do it.
Have a look at this picture.. As you can see the cables are running slightly under the front seats and not just above the seat fastenings.
Right I will start at the front, there is a bolt holding your dash in place, id advise to remove this because you will find it easier to work with the little space you have and you will be able to pull the carpet up easily. (This bolt can be tricky to put back so try not to get frustrated)
from the next picture you can tell where your RCA cable and your Remote cable should run behind the bolt and up in to the dash.
Next up you will need to remove the front seat, have a look at the space in the picture, do you really think it would be wise to try run your cables without removing the seats?
Here is a picture front the back of the seat; ive added a red line where you should have your cables running.
Next you will need to remove the seatbelt fastener to run the cable through to the back of your car. Have a look at this picture.
Once past this point you would simply pull the carpet back and run under the back seat, you would need to remove the back car seat, there is a small latch to hold the back seat in place if I remember right its a star screw, remove that so you can easily pull the carpet back and run your cables.
Have a look at this picture; this is where your cables would come out.
Now you would simply run the cables through to the boot, you shouldn’t need to unbolt anything, but if I was you id unbolt and remove the rear seats back rest.
This was for the driver’s side of the car; you would do exactly the same on the passenger side of the car.