There is only one full time live in a mk2b punto at the stereo loom and its constantly live so even with key in your pocket its still pumping volts, ie your radio runs the risk of flattening the battery over a period of car non use or worse some car stereos get very hot if left with permanent live going down the yellow and red cables to the stereo because you joined them both up and could cause a fire over a longer period of time it actually says in many instruction manuals not to join these wires, last one i saw a month ago was an expensive pioneer manual so obviously manufacturers are pretty keen you dont do it.
The other risk is the factory stereo works by talking to the main control unit to confirm that its the right one ie they are coded together on whats called a can bus connection these cables send messages rather than 12 volts so if you dont leave these cables going nowhere when you connect up your aftermarket stereo you run the risk of sending voltages down the wrong wire, first thing you will notice is the yellow light of engine management and when code read this always comes up as a speed sensor fault, caught early you can clear the code disconnect your errors and car will live another day.,i know this by having done it by accident .

I break em to fix em, sometimes, so my advice is as follows for a later 2b wiring loom equipped aftermarket stereo

Buy a connecting iso lead and face plate plus obviously the aerial connecting pin
Locate the one fulltime 12 constant power lead on the car loom and make sure this goes to unswitched aftermarket stereo lead usually the yelow lead I use a multimeter but a bulb would suffice (ignition turned off)

Piggyback ideally by soldering a new power cable from the switched live on the cig lighter ,you will need to do some dismantling to get to this but its in the guides, run this new lead in some washer pipe as it goes past the crash protection cage for the dashboard which is very sharp in places, connect this cable to the red on your new iso connecting lead
You should now find on the power lead on your new iso lead you have only 3 wires connected to the radio itself ,a yellow, a red and a black all other coloured cables from car iso now hopefully go nowhere so no chance of any backward power going down them
The other iso plug lead is all speaker connections and thus doesnt need touching

You will find its hard to put the radio in the dash when all connected up because the hole wasnt designed for an aftermarket unit ,persevere and try to feed as much loom down the back right hand side hole before sliding radio in,thin hands help here
Your radio will fit if you go carefully
Convert the standard aerial connection from ISO to Din so you can connect an aftermarket car radio these are available for 99pence on ebay whereas last time they were £5 each in maplins or halfords when i looked just like the iso leads are much cheaper on ebay too so a little planning saves a lot of cash