cabrio sub+amp on hk (how to wire) URGENT

Got an Hardstone HS Z102 amplifier plus a very "good looking" 25cm subwoofer (Hardstone but can't remember the code)....

How to install it into a trunk of a cabrio that has already hk sys???

i've already took a look to this post ----->this 1

....but listen..: in my coupe' i had the same amp wired to the speakers cable by the hi input on the ampli (nav tv... so no rca...)

i've tried to use a line converter (to emulate an rca input) but horrible interference comes from the speakers....

so i skip the line converter....

I understood that hk has 2 ampli (one for the sub and one for the other speakers).....

My question are:
1)where are them located?
2) which cable do I have to cut to disable the stock sub amp.???
3) will I wire the ampli to the existing hk sub ampli?? (or i risk to overload the system)

for those that ignore what HI INPUT is :
Hi input is that kind of input that allows you to connect the ampli to a non-amplified (w/out RCA) stereo.. using the stock speakers cable....


sorry if my english isn't good enough to explain it well as i think it in my mind........


that's urgent!!!!!!