[akhter]

there is also some stuff to worry about on the center. the center signal my have already be crossed over from amp (ie. not full range) so you cannot use the single mid signal and split. you also cannot use the amp L or R signals. so you need a signal summing device out of the full range L+R. you also may need to use something like JL audio cleansweep to flatten the signals (if they are not flat on base hifi). i had to do this on my e90 because the crappy hifi speakers were so bad that bmw had to eq them from head unit.

if i were u what i would do is leave the center alone or replace with a coax 4 inch instead of adding a tweeter as it is too complicated.

upgrade teh doors and add a simple 4ch amp (like jl audio). if u want better, add a cleansweep to flatten the signal. if u want more then upgrade the center to coax and use a signal summing and use the 3rd channel of the 4ch amp.