VS Caprice Stereo Change

[Manolz]

Hi, so my 10 CD stacker in the VS Caprice recently stopped working as everytime press CD on the eurovox stereo it constantly shows E-99 (yes ive looked up the error in the manual and the solution- holding on/off for 6 seconds- only worked once for 5 minutes, now it wont work at all) so i decided its time to change the HU.
I bought ISO connectors for the car and the Sony HU i had lying around from my last car. So ive gotten to the back of the stereo but there is one thick cable coming from the car somwhere going into the stock unit that i cant easily just pull out. I wanted to have it all replaceable (thats why i got the ISO connectors) just in-case i ever wanted to put the stock stereo back in and if i were to cut this cable i doubt id be able to solder it all up again.. So can anybody tell me what this cable is for and if it is really needed?
I do have a picture of it that i could upload if really necessary.
Thanks in advance

[wamboin23]

will be either aerial connection, or connection for 10 stacker in the boot.

[Manolz]

yea im guessing it must be the stacker. I have a problem though, i plugged in the Sony HU into the car using the ISO connectors and only 2 speakers are actually working. Is there any easy way of going about this? or am i going to have to cut it all off and solder them together manually... (which sort of defeats the purpose of me buying the ISO connectors.