Sportcross is300 audio head unit upgrade

[Colbat]

Hi all, new here and with an is300 sportcross with stock speakers. I am looking trying to get my head around something here with aftermarket head units and wiring.

At the moment I have a Sony head unit with a harness adapter to the stock amp, I'm happy with how the stock amp sounds however I cannot get all the speakers working at the same time, I've managed to get the front and rear pair working or the front and sub working, but not all five at once. 

I'm not after changing the amp as it does a pretty good job for filtering frequencies to each speaker for the tweeters and main speakers, and I do not want to install a separate subwoofer in the boot, the stock one does a pretty good job considering the cars age and that it's stock!

 

Any help with how to get around this issue would be greatly appreciated, many thanks 👌