Amp installation

[Grah]

Hi, trying to install a sub/amp in a none Bose car, i took the input for new amp by taking output from the sub output from the original amp; the amp is switching on and I'm getting sound but no where near what it should, I suspect the input signal is incorrect?

[VAG-Abound]

Are you running the (usually blue?) wire to it that says when head unit is in use, and amp is actually needed? Does it have input for one? Is it sounding loudish, bit not bassy? Something else? I'm also wondering if original system needs a VCDS / vehicle coding to say sub is there and for original amp to use it.

EDIT: And while heading off to check that VCDS thing there seem to be multiple threads on challenges of using aftermarket parts.