Finally after many days, my subwoofer install is more or less complete!
After removing trim from half the car I got access to the rear bulkhead where the original factory fitted subwoofers fit. The idea was to make an enclosure from that space. to do that I used sound deadening to seal up all the holes and to give better sound from the subs:
Next I mounted a MDF panel for each side secured by the original mounting points for the factory subs and some strong automotive double side adhesive tape on the left and right sides. I then covered the exterior surfaces with more sound deadening to seal any gaps or holes and put some acoustic foam on the outer side of the MDF so it makes a good seal with the car's trim panel:
I then fed the wiring for the subs by drilling a small hole in the MDF. These subs are DVC 2 Ohms so wired them in series parallel to give a 2 Ohm load at the amp:
For the amp I made an amp board from MDF and covered it in subwoofer box carpet which I had laying around onto which the amp is mounted together with it's external fuse (this amp has no internal fuse) and cabling:
And here is the install all completed:
I also mounted a bass knob in the dash pocket on the driver's side:
Having completed the sub install I decided to make sure the gains were correctly adjusted on all the amps using a cheap oscilloscope and some test tones, to make sure the amps aren't clipping but giving out their max potential:
So the end results? At first they didn't sound right acoustically, no doubt due to not having a proper sealed enclosure made of MDF. Not sure how to describe it but they sounded a bit echoey if that makes sense. So to eliminate that I decided to put some polyfil in the enclosed space and that actually did the trick and now they sound great! These subs for the tiny size absolutely pound! Obviously they aren't going to rattle the windows or anything but are capable of putting out bass you can feel and they make a huge difference to rest of the sound upgrades I already made. For what little this system has cost me I am really chuffed! I wonder how well it sounds next to a Z4 with factory premium sound system? Now I just have to make some final adjustments to the crossover points and maybe bass eq on the amp and I am all done....... well at least for now
still it won't be a patch on your system...I honestly could not be arsed with that 
Scottish Borders
