Need help with common ground.
I pulled the stock deck out. Wired up the harness and thought i was ready to go. But because i have the BOSE system i have a common ground for the speakers. So please can some one explain to me what needs to be done. So far ive heard to ways: one is to rewire the whole speaker system. Or i can splice into the ground somehow and ill be good to go.

so please let me know the fastest and easiest way to do it.
thanks