Audio System Install (cont.)
The next morning I attacked the rear speakers. When the stock rear speaker is removed, you will find a thin piece of gray foam in the side of the enclosure. When that is removed, it's obvious that the space behind it extends all the way to the rear cargo area. This is not good for an acoustical enclosure because it's 1) not airtight, and 2) oddly shaped. I went to a fabric store and bought some 5" foam rubber to fill that void, closing off the passage to the rear. I compressed the 5" thickness into a space about 1" wide, so it took a little work to get it jammed in there. But it's sealed off real good now! Over the foam I applied some membrane to make it more airtight. That was followed by speaker polyfill, and membrane on the speaker hole edges. I had 6.5" Infinity Kappa 3-ways to install here, so I got some adapter plates from Crutchfield rather than making them myself. These went on and were also trimmed with membrane. I ran the new Monster speaker cable throught the rubber grommet next to the speaker hole. Finally the speakers were installed and the rear was done.
Next came the amps and subwoofer. I was determined not to lose any cargo space to audio equipment - the MINI has little enough as it is. So after getting the dimensions of the cavities that are located under the carpet beneath the front seats, I looked for equipment that would fit there. My original plan was to drive the rear speakers directly from the head unit, so I bought a 2 channel blaupunkt PA-275 T-class amp to drive the fronts. It was right after I bought the amp that I found a great deal on a top-of-the-line CDA-7998 head unit from Alpine, so I snagged it. The problem? There's no amplification in that head unit - it relies completely on external amps. So I had to get another PA-275 amp to drive the rears. It was ok though because the amps are so small they both still fit in one cavity
The subwoofer is a Clarion SRV-303 self-powered unit. It's the only sub I found that was compact enough to fit in one of these cavities. It's not a powerhouse (120 watts max), but I only wanted some low-end fill so it turned out to be perfect.
At this point I hadn't taken the carpet up - I couldn't figure out how to remove the center console with the emergency brake in it, and I didn't recall seeing a how-to for it on the forums. So to get to the cavities I just lifted the carpet from each side and propped it up with styrofoam so I could work in the cavities. The amps are small perforated metal boxes that come inside an enclosure. The bottom half of the enclosure is a fairly thick metal tray, and the top half is a dorky colorful plastic shell in case the amp is visible. I tossed the plastic shell, but kept the amp screwed into the metal base to act as a heat sink and to isolate it from any metal in the cavity. Here's a pic of a test-fit of the amps in the cavity before wiring them up:
Then I lined the cavities with membrane to isolate them from the floor and prevent rattling, and stuck the amps down with double-sided mounting tape. Next I needed to get power to the amps. I had purchased an amp installation kit which had an 8 gauge power cable with an in-line 50 amp fuse, ground wire, and various connectors. I looked at the location of the battery in the bonnet and found that right behind it in the firewall above the steering wheel there was a rubber grommet that already had a big taped bundle running through it. There was plenty of room in the grommet so I poked a hole through it and fed my power cable into the hole after taping it to the end of a piece of coat hanger wire. I pulled enough cable through so that the end with the fuse holder was in the right place to hook up to the battery.
View of cable (red) through the grommet from the engine:
And from the interior, looking up under the steering wheel:
On the inside I ran the power cable down and under the fuse panel and along the side under the door with the other wire bundle that was already there until it reached the cavity. I terminated it into one end of a brass power distribution block that I had bought at an auto parts store. From the other side of the distribution block I attached power cables to feed each of the two amps, the subwoofer, and the rear power outlet that I figured I'd install while I had everything apart. You can see the distribution block in the lower left portion of this pic:
While we're here, here's the rear power outlet: 
The red cable on the left is from the battery, the 2 red cables coming out on the right
power the amps, the yellow cable powers the subwoofer. I hadn't attached the cable
for the rear power outlet yet. The lighter blue cable is the remote turn-on
lead from the head unit. The darker blue cables are grounds for the amps. The
grounding point is just out of view on the left side of the picture. I sanded the
paint off a spot on the side of the structural beam that forms the front of the cavity and
grounded the 2 amps and subwoofer there (the ground wire for the sub is not attached yet
in the picture).
The routing of the RCA feeds and the speaker cables is hard to explain, but here's a diagram that may help:
The RCA feeds for the front and rear start up behind the head unit, go under the carpet on the floor on the driver's side of the center console, and then to their respective amps on the opposite side from the power cables. Since I decided to use the stock wiring for the front speakers, I connected new speaker cables to that wiring behind the head unit and routed them the same way to the front amp.
The cable from the rear driver's side speaker is routed under the back seat over to the passenger side where it's joined by that speaker's cable. Both cables are then routed under the passenger door to just in front of the passenger cavity location, where they go under the carpet in front of the cavity, up over the center hump between the gear shift console and the emergency brake console (still under the carpet), and then to the rear amp. This roundabout route was done solely to avoid the power cables. I only wanted to take this car apart once, so I figured I should do everything possible to avoid the noise problems others have had.
I lined the passenger side cavity with membrane and stuck down the subwoofer with double-sided mounting tape. I snapped in it's power/ground/remote turn-on connector and ran those three wires under the carpet that goes down vertically from the rear seat, where your calves would be if you were sitting there. Here you can see the power and ground (yellow and black) cables to the left (the blue remote turn-on wire is not in view). You can also see the rear speaker cables routed across the bottom of the picture (under the door) and then up the right side of the picture where they will go over the center hump as previously described. The RCA feeds run under the carpet along the center hump then up behind the head unit. The bundle of thick black cable in the foreground is for the remote crossover/volume control for the subwoofer which I later mounted in the left side of the parcel shelf:
