Looking to do some audio upgrades but don't want to lose the satnav,

is this possible and how

You could upgrade the speakers and sound deaden the doors with Dynamat (or equivalent).

If you want to use amplifiers then you could get a JL Audio CleanSweep to convert the speaker output to a clean RCA signal - don't be tempted to use cheap speaker to RCA convertors if you're looking for high quality sound quality.

aswell as the JL clean sweep there is the Audioson Bit 10 and JBL MS8