part required for: fityting aftermarket headunit with satnav/harmon Karmons HELP??

[chris*locke]

my mrs has a mini cab with harmon Karmon speaker system & satnav fitted as std.

what parts to i have to purchase to fit the headunit so everything still works as it should???

i know there is a interface unit to get the satnav sound to still work but is there anything else?????????
as im sure ive read somewhere another interface box is required for the speakers/amp to work???

thanks

[Jaks101]

Did you have any joy with the connectors as I'm struggling.