The complexity of the car electronics has really nothing to do with an aftermarket audio system. Only Volvo's own radios are connected to the car's main computer system. That's why you have to get software updates for everything you add to a Volvo radio, because these tell the radio what to do. Basically this is just another money rip-off. I don't see any reason why the Volvo radio would separately have to be told whether it has eg. a CD-changer connected to it or not. Any aftermarket system notice this by itself.

When you replace the Volvo radio with an aftermarket one, you just have to get and adapter wire between the car's audio connector and the standard connector (DIN?) that the aftermarket units have. These adapter cables are available from aftermarket brands and Volvo. After this the power inlet, antenna, speakers etc. are available for the aftermarket unit.

The steering wheel buttons will however not work. At least I haven't yet heard of any aftermarket manufacturers that would have made an separate adapter for Volvo's steering wheel controls.

-Qube-