The Install
Plan and plan well (Visualize the entire system)
I planned to line convert each speaker output (8 total) from the HK amp and amplify each. The problem here is that I also wanted a sub woofer which meant that I needed another output ( combined or summed). After checking all outputs I found the two rear speaker outputs (I had heard rumor of them), I combined the two dual coil outputs into a single input (twisting the two inputs into one input - Yes it does really work). This allowed the eight HK amp outputs to now be six. Being that there are right/left for each channel I needed a total of three (3) line converters to go from speaker output to line inputs. Contingency plans said that if the Tweeter combo was not combined I would need a set of passive crossovers for the mid/tweeter or door/tweeter combo. I ordered a/d/s crossovers from EBAY. I also order a second four channel MTX amp from EBAY. I had originally planned to use the factory speakers (that changed later).
The amps placement can be an issue. Under the seats / in the side panels / false boot? I trial fit the amps I had under the seat but was unhappy (too tight without cutting carpet - which I did not want to do). I decided on the false boot after seeing great examples of other installs. Thanks.
All splices can be done in the rear boot area. No runs up to the front HU are necessary. I kept everything in the boot area. This of course did not include running the power wire from the battery area (located by the way for non-S persons in the front engine area). Ground cabling should always be done as close to the amps as possible. Each amp needed it's own power / ground so I purchased two splitters (common blocks). I only bought 8 gauge wire even though I was running two amps. History has proven to me that I've never had a problem with the current draw, even at high operational volumes. I also physically felt the wires upon hook up to verify that fact (no heat). Not the least bit warm. Leave enough wire loose to be able to lift the false boot bottom up and out of the way (remember that tire and tool thing in the boot for non-S persons again).
You will have (8) out sets (+/-) and (8) input sets (+/-). Buy from Radio Shack the screw connector strips. It makes life easier. You will have to extend the output wires if you wish to place the line converters in the far left -hand corner (that an accessible part of the boot). Use good crimps or solder the wires. I recommend the line converter placement there because, even though I did not have to, you might have to adjust the output of the line converters after reassembling the plastic side panels.
HK amp speaker outputs (cut at plug) go to the line converters which RCA line feed your new amps.
Out of the new amps the speaker outputs go back to the original (cut at plug) HK speaker wires which of course go back to the original (or replacement) speakers.
Test the speaker connections with a 1.5 volt battery. This an old trick to get a clicking sound out of a speaker (and check for directional movement). Plus or minus does matter. Check all proposed speakers and their wires. Be sure you can account for all wires.
You do not have to cut all of the wires (only speaker wires).
Draw a diagram if you need to.
Pre assemble the project on a bench.
Cover all interior areas with towels/sheets/etc to prevent your tools from scratching the surfaces. Be mindful of rubber gaskets hanging out of their guides (don't shut doors on them or don't sit on them). That I imagine could get costly.
You'll need a set of TORX driver tools.
Cover the screwdriver/prybar tool tips with electrical tape. This saves the finish of the vehicle.
Have dremel tool available. It will save you time after time.
Note: All of this wordy discussion will become clearer when you see the pictures.