Cables : You need 2.
1. Search eBay for 'KKL'
If you're good at soldering, then it doesn't matter if it has a switch or has been pre-modified - you can do it yourself.
2. You need a adaptor cable to go from the 16 way socket of the lead above to the 3 way connectors you have in your car
search eBay for Fiat diagnostic adaptor cable
To modify the first lead. At the crudest level you connect pins 1 ( ABS ), 3 ( Airbag ) and 7 ( Engine ) together. That's it really, just two bits of wire and a couple minutes of soldering. The pin numbers are moulded into the plastic as well! If you want to go fancy, you can wire in a switch to select the connection. Some say that having all three connected can trigger an airbag release but I don't really buy that as the triggering is done by way of deceleration devices.
Next you need a laptop with a USB socket. I use a fairly average Netbook. When you plug in the first cable, it should install the device. Once it does that, you're pretty much ready to go.
Plug the second cable into the first and the second lead into the car's engine/abs or airbag connector and the black & red crocodile clips need to be connected to +12v & ground - you can use the cigarette lighter socket in a P1.
Now the gotcha. It is possible that the laptop's port ( USB ) configuration may need tweaking.
What you need to do is on the FiatEcuScan page here :
FiatECUScan - Fiat, Alfa, Lancia ECU Reader / Scanner
Finally, your engine ECU is probably a Bosch M2.10.3 ( the M2.10.4 didn't go into cars until April 1997 ). Once you have the software running, turn the key to ignition and try and connect.
Hope that helps!