Lots of questions...
In order of recollection...
There will be a hose about 8" long sticking into the engine bay from the firewall. It looks like black corrugated wire loom. On the end is a funny cow udders looking rubber cap. About 17mm in diameter if I remember right. The same tube can be found in the utility area above the driver's feet. Remove both ends, slide your 4 gauge (depending) through and viola! Simplest EVER!

Ground from the din: Use pin #2 <> Let's see if I can get this diagram to come out...

1 5
2 6 4
3

1 Left rear
2 signal ground
3 Right rear
4 Right front
5 Left front
6 Amp trigger (+12v switched)

Speakers: Speakers are 6.5" components in front and 6.5" coaxials in rear. Tweeters in the rear pillars. speaker depth is not an issue but you will have to cut the cute port off the backs of each mounting shell. I have 6.5" Diamond Hex in all 4 factory spots. The dashboard tweeter loaction is obnoxious in my opinion. The reflection from the glass yields an unpleasant harshness.

Wiring: Are you adding an aftermarket 4 channel amp or a Volvo factory under the seat-type? Either way you should install using the factory wiring up to the point you need to branch off for the crossovers if component speakers are used.

I am in the car stereo bizz. I have a pretty advanced system in my R. Uses the SC900.