Doug, my understanding is that the audio head unit in the dash is the same for both standard and premium systems. The difference is that the standard unit feeds its six speakers directly while the premium set-up has an amplifier in the boot, two additional tweeter speakers and, for the coupe only, a subwoofer in the parcel shelf.
Looking at the wiring diagrams, it appears that the eight wire harness that in the standard arrangement connects the CD changer in the boot to the head unit is re-used to connect the head unit to the amplifier. The amplifier comes with a fitted connection for the CD changer. If you happen to have a fitted sat nav, there's a bit more, but not much. Speaker wiring might need to be run from scratch
Model years up to 2004 (?) had the option of a hard wired phone that parked in the centre armrest, with the control module situated under the centre console and connected to the audio head unit. For 2005, this was modified by disconnecting the handset and fitting a Bluetooth module. You'll have to check what the wiring arrangements are in your centre console then look to getting a phone module and a Bluetooth one ( I see one on E bay today!)
https://www.ebay.co.uk/itm/401713616778 ... Swv-ZaM67K
You don't say which Model Year you want to modify and I've based the above on MY 2003-5 and Alpine equipment, but the earlier arrangements look much the same. You will need the relevant wiring diagrams for your particular X100. If you haven't got them, just say and a link can be provided