Kanga, it's the easiest job I've done on my Spider to date!
1. Undo the gear lever gaiter and pull up the gear level surround. This will give you access to the wires running under the head unit.
2. Undo the screws in the footwell both sides that hold the panels on either side of the central console area with the head unit - this will give you more access to the wires behind the head unit.
3. If you don't have pullers for the original head unit, just snip a wire coat hanger to give you two pokey things, and poke them down the holes on the head unit. You should then be able to slide the head unit out.
4. Disconnect the plugs and the aerial
5. Remove the "cage" that the original head unit was in (assuming you get a new one with your new unit)
6. Loosely fit the new cage (but don't lock it in place until all is tested!)
7. Connect all plugs to the new head unit and test it
8. When all's OK, do a trial fit of the head unit. You'll have to pull the wires from the back as you slide it in ... it's a cosy fit. That's why you need good access to all the wiring!
9. Once you know you can do that OK, slide it out again and lock the cage into place. Then refit.
Only funny is that there's no switched ignition, but I just pop the face off my head unit and then it's not using any power anyway. There is a wiring mod you can do to give you a switched ignition feed to the head unit - posts on here somewhere.