I'll have a go at describing how I did it. I have a vz2 so I have a spare wheel.
Remove parcel shelf
Remove boot floor
Tilt forward the back seats
Lift out the foam blocks from under boot floor, they do come out as 1 piece, lift up the part furthest away from you and slide it all away from you then lift.
Remove current light, easily.pulls.out.
Attach new cable to old light and old cable to new cable, dont push it in the hole yet.
Remove the cross trim (5 in the below image) the cross trim needs to be prised up, slide a trim tool under the trim at the boot rubber and prise it straight up, work your way along the trim prising as you go, be careful this has a cable attached but there's enough slack to put it to one side.
The carpet on the sides of the boot can now be lifted a little.
I used a cable rod from the back of the boot under the side carpet up to the hole where the standard light is, attached new cable to it and pulled it out.
Run the new cable under the cross.trim, taping it to the boot floor.
Prise with beast mode the blanking plate for the new light.
Get cable rod with flexible end, tape end of new cable to it and push it under the carpet to the hole.
Attach new light and clip in place.
Replace all.trim is a reverse of the above.
Hope that helps, and makes sense!