I've read that some owners have used that space to locate a CD autochanger, others have put them under the front passenger seat. Not sure about individual models, obviously you'll need to check dimensions/clearances first.
Thinking about fitting one to my standard HU605 unit when funds permit, so would be interested to know which unit you fit, where you decide to fit it & how you get on with wiring. Good luck
Volvo V40 1998 - SC-805 head unit - Alpine CHM S630 CD changer.
adapter/interface required was a VOLA2
There is a space at the back of the spare, but it was not level enough to fit onto the floor.
So I suspended it in the same area, bolting it under the lift up floor cover.
When cover is lifted it is easily accessible for changing magazines.
The harness/cable threads fairly easily (left side used)under the rear seat side padding and along left side under carpet to dash.