Ah ok thnx for answering all my questions
So..to replace your existing 6 speaker should be straight forward with the following caveats, most of which your fitter should/can/has to cope with!
If you are using the existing head unit and existing wiring then most probably you will have an ASD (Active sound design) box in the boot, on the LHS of the rear bulkhead behind the left seat hidden by the boot liner panel..which is easy to remove /access once you've done the clips that retain the top front section of the cargo net..
IF you have that box (many say they don't have it, but I think its fitted to all 18i 20i 28i) and the 4 cars I examined all lhad it..then..
The issue is that power and outputs from the headunit route through it and then onto the speakers.
If you remove the ASD unit the headunit is disabled and you lose speaker connectivity..
SO if you have the unit, two options, a) hand cut and reroute the speaker inputs into the ASD and tie them to the speaker outputs from tthe ASD to bypass while still keeping it powered up to avoid fault codes and keep the headunit alive or b) by a bypass harness from technic pnp in the states..the same cable is used on MIni and 3 series etc.. if no ASD then ignore this para..
If you are using the front footwell speaker positions for your woofers be aware by default that the wires from those speakers are wired in SERIES on ONE channel normally..the intercept point for the conection to turn it into two channels is in the RHS of the boot bulkhead next to where the DSP/AMP would be if you had the 11 or 14 speaker system.
From what I can discern your mid range speakers will not fit the door cards without some butchery..your installers issue to make that happen /make neat etc..
Tweeters should be easier assuming standard size..
Woofers will need an adapter ring fabricating to again compensate for hole size /non standard palcement of screw holes as per door card..
Amp wise if its a 4 channel amp then the output from the headunit is non standard 2.5v ish as opposed to 0.6v.. but you can adjust the gain on the amp is assume if requuired to avoid overdrive and destroying the tweeters!!
Your issues is how you drive the speakers you''ve got, not clear how many tweeters /mid range and woofers you are proposing?
Then with 4 channel amp whcih channel goes to which speakers and whether they are used actively or passive? pls advise what you plan?
Should be a dramatic improvement in quality and 'punch' as the base steruo is only about 15w peak /music power per channel!
RSVP!
