Do a search on here re. the amp, you will have to snip one wire on the factory loom that goes out of the factory hu and to the factory amp... and add an AV connector/end so it can plug into the aftermarket hu.
Offhand i cant remember which wire it was, (green maybe?) i remember I bought the AV ends that have a grub screw so you can put the wire in there and tighten it down. Alternatively you can just run 2 new AV cables from the hu to your factory amp along/under the sill covers. (Series 2's only have 1 AV wire hooked up but all the amps ive seen have the 2 jacks on them.)
Ive put a single din hu in the satnav fascia using the aerpro single din plastic that I bought on ebay, bought their loom also to do the job and my factory steering wheel buttons work with a pioneer hu, now i have bluetooth, hands free phone etc etc.
Although my car wasnt a factory satnav car so cant help you there, ive put a satnav fascia in it I got from an LTD and a different screen in the satnav part.
around 13 seconds you can see the single din hu...
https://www.youtube.com/watch?v=hQ3BoEamrao
I like the extra storage space I have now, and it sounds better despite the factory amp and speakers remaining.