leadfan Car stereo Wiring Method(No need for Wiring Harness Adapter)(Need Circuit Tester)

Suitable for all Naify single din & double din car stereo
Need Circuit Tester

[@polimarco5994]

In my car I have a single wire for the 4 negative poles of my speakers. Can I connect all the 4 negative wire for the speakers in that single wire on the connector?

[@dandyd543]

When finding the ground wire, 2 wires produce 12 volts on my multimeter. How do I determine which is ground?

[@JaguarMan5280]

The battery method did not work. My speakers are powered by an amp. How do I locate the amp wire from oem wiring harness?