well you dont want to get invoklved

otherwise id have said false floor on boot lifters (to get to the spare) this would be nearly invisable but a lot of work
so to do it simply... see if you can move the changer up a little because on the mk5 there is a big recess just above where your changer is and where it is, its likly to get smashed
id also attach the amp to the sub box and put that on the passenger side (weight distribution

) and also tie it down because if your in an accident at 50 mph that thing WILL rip the rear seat clean out of its mounting and then use the seat to pin you to the wind screen. i use 2 straps with a ratchet in them to hold mine down. they are attached to the luggage securing points and are strong enough to tow a mk3 golf (tried and tested

)