Need Help with car stereo bluetooth mod!! ALOT OF UNWANTED NOISEY INTERFERANCE

[D.gee478]

Hey guys, new to the site and am hoping there is someone here willing to help point me in the right direction.

I'm currently building a car and being the tinkerer that loves all things electronic but am by no means anything close to having a degree in anything alike as im just a dirty concretor

I have decided to try modifying my cars factory headunit to have blutooth As I want it to all look original.

COMPONENTS.
Stereo - Eurovox MCC-6329
Blutooth Module - cheap ebay 12v BT07 Module
Pre-amp chip - LA3161

So far I have managed to wire in a blutooth module from ebay with

.12v+ to the casset players motor possitive wire
.-ground to stereos casing
.left/right channel & ground i connected to the 3 corresponding outputs on the preamp ic chip I think it is

And am currently getting alot of unwanted noisy interference and the sound is not that loud.

How can I distinguish between
1. ground Loop
2. Connected to the wrong part of the pcb
3. Is it my Regulated DC power suply i have been using to power my headunit on a test bench?
4. Any other reason for the noise?

I don't know where to start. I have posted wiring diagrams pics etc if anyone can help would be much appreciated. Thankyou

The stereo manual I have is too large to upload.