Amp installation help needed

[jaynana]

hi, i have a 2004 X5 with nav, cd changer, no dsp. and i'm trying to install an amp+sub(JBL).

i've got power, ground, signal connected all ok.

issue is when connecting the sound/signal to the amp (RCAs)

i've got the speaker outs from the factory BM54 as below:
http://www.xoutpost.com/articles/x5/...formation.html

so basically the rear right + rear left.

when i connect one of those to the first RCAs inner the sub starts working; but the moment i connect the first RCA outer wire itself the radio knocks itself out.

and i have to remove that outer, and restart the radio to get any sound from it.

amp works if i connect only the inner cable of the RCA but that's no good because then it gives out ambient noise (i believe because then its not properly grounded?).

can anyone help me understand what i'm doing wrong here pls?