I have a 2003 Volvo S60 and need help finding the reverse wire. I am installing a D3 with back up camera.

In the trunk of your vehicle the leads going to your backup lights is the easiest way to find it.

I'm claiming ignorance and stupidity up front; are you saying that in order to have the headunit display the rearview camera, all we have to do is to connect the 'rearview wire' to one of the backup light wires?

yep ! that's it !

that's the easiest way in that car.(that's the easiest way in most cars)

unless you want to go here,Blue wire at reverse light relay on REM

The REM (Rear Electronic Module) is part of the fuse/relay box in the driver side of the trunk.