Amplifiers: If the amplifier is small enough, try to install it under one of the front seats. Or possibly in the trunk, but screwed into the backside of the rear seat.
Some people have gotten away with mounting the amplifier upside down, screwed to the underside of the rear deck. (Rear deck= the flat shelf behind the back seat, where the rear speakers usually sit.)
It would be wise to do some research on your particular subwoofer, so that you can build the smallest enclosure possible.
Many people have sacrificed their spare tire, and used the tire well as a subwoofer enclosure----either by simply mounting a board over the hole, or doing more elaborate work with fiberglass.
There are a few brands of infinite baffle subwoofers, which require no box----they mount to the rear deck just like the 6x9s. Kicker and JL Audio, for example, make such woofers.
If you do put your woofer in a standard box, use quick-disconnect wiring, and make the whole thing easy to remove, for when you need even more cargo-carrying capacity. (For this reason, don’t mount your amplifier to the speaker box—there would be way too many wires to disconnect each time.)
If you visit carsound.com, do a search on “AP Mat” or “Aperiodic Membrane” — it’s an uncommon way of installing a subwoofer without a box. I haven’t attempted such a setup before, so I don’t have an opinion on it, but it sure is worth looking into. Richard Clark is one of the writers for Car Sound magazine; he seems to know a lot about AP setups.
Here are some useful links for you:
www.carsound.com: The discussion forums are filled with intelligent people, many of them working for manufacturers. I’m a regular there, too.
www.sounddomain.com: The discussion forums are mostly newbies arguing about nonsense. But there’s a wonderful section where people post descriptions and photos of their installations----even if you don’t find a car like yours, you’ll get loads of new ideas.
www.the12volt.com: Highly intelligent site, with lots of common-sense explanations of how various car audio and security things work.
P.S. Just took a quick look at that Holden page you linked to…seems like quite the car, and I wonder if any are around in the U.S. With the untimely demise of the Chevy Caprice, America only has one real sedan left–the Ford Crown Victoria. It’s nice to see that elsewhere in the world, you can still get a good sized car with a good sized engine-----is it rear wheel drive too?
, But anythings possible I suppose. I’ll have a look at those links you posted.