Been there and done that.... Had 2 x 1000w Subs in my kit car with 4x500w mids and some tweeters... Was loud as hell lucky for me it was fiberglass as there would be little rust holding it together if not.
Got a CD player in my 300tdi Disco which gets used once a blue moon, Not sure what is in the V8 as I have never had it on as the engine is music to my ears.
And not put the 200tdi Defender one on yet either (Well to test it worked when I got it!) spend most of the time listening for the next job to fix! or where needs work to stop rattles or squeeks.
Have the system in a box waiting to be fitted to something but its a little like LPG... I got the disco as it is 7 seater, to fit either a system or LPG just cuts into this space!