Hello forums,

Considering doing a major upgrade to the sound system in my BA 05 fairmont, and am seeking your advice (and approval). I already have a 12" Type-S sub on a monoblock in the boot hooked up through line level inputs, but the system just isn't loud enough for my liking (gets very distorted at high volume) and so I'm looking at revamping everything. New head unit, 4 channel amp, ditch line levels, 4 new speakers etc.

Before I start buying stuff, thought I'd clear a few things up first. I've heard all speakers in the car are 5x7's. Has anyone had any luck putting different size speakers in, like 6-1/2" in the back and 5-1/2" or even 6-1/2" in the front? For the front I assume it's just a matter of what you can squeeze under that grill, but the parcel shelf would be more problematic. Is there an easy way to widen/fill the gap where the 5x7's were? Failing all that, will a decent set of 5x7's with the 12" in the boot still sound okay?

As to product, I'm going to get the 4channel Alpine MRP-F300 (I'm sort of stuck with this, since I already have the MRP-500 and am dangerously OCD about symmetry). Head unit I'm less worried about, Alpine gear looks nice but is a bit expensive for my liking. All I want is something with two pre-outs, iPod usb and the steering wheel remote compatibility. The cheapest alpine HU with this is the CDE-102Ei at $350 at dealer, or $250 on ebay. A kenwood equivalent, MP443 is $200 on ryda.com.au (i've heard this site is pretty good). Speakers I was thinking Alpine Type-R (bit expensive, they don't make Type-S 6x7's) but that's only because they are the only brand I have any real experience with. I'm going to test some other brands out soon, Kenwood, Focal, MB Quart etc.

So that I don't lose my rear parking sensors I'm either going to reroute the old factory rear speaker wire to some random cheap speakers and hide them somewhere (kickpanels, under seat maybe) or run them into a line level converter and use a y-split RCA to connect them to the rear speaker channel of the amp. We'll see.

I'm toying around with the idea of adding a second Type-S into the boot, and making custom boxes for each (the one I have is in some ugly thing i bought at autobahn), putting one on each side of the boot. My worry here is that each side of the boot is a slightly different shape, due to the fuel cap on the left hand side. If I make two boxes with an identical volume (through use of extra MDF filling/polyfill), but are slightly different shapes, will they produce a different sound? Will they interfere slightly with each other?

Further, with the back seats, I can't put the middle seat down on its own, all I can do is either put the left down, or put the middle and right down together. Usually what I do is just put the left down on its own, since it doesn't look so bad and I retain two seats, but if I put two subs in the back facing each other, will there be a problem when one sub is closer to that gap than the other? Or due to the bendy abilities of low subwoofer frequencies, does it not matter?

All those Headunits I listed have 'steering wheel remote compatibility' with third party products. Has anyone used any of these? How expensive are they and do they work? Also, are they hard to install in the BA?

So what do you think? I know there's a lot of questions there, but I would greatly appreciate any help people can offer.