au2 ghia sub and amp

[eb falcon gl]

hi, i recently had a new alpine headunit installed into my au2 fairmont ghia with stock speakers, sub and amp.

But i think there is something up with the wiring with the sub, amp and rear speakers. Its like they are all connected to the one thing. eg when i change the settings on the headunit like turn off the sub it will stay on.
however if I unplug the rca connector at the amp the sub goes off, but the rear speakers stay on.

Has anyone got any ideas on what to check or how it should be wired up?

thanks for the help