I'm not familiar with the '69 units, but I assume it's still a mono AM/FM like my '67. If so, the dash speaker switch is not really a fader control. It just cuts power to front only, rear only, or both.
When I ordered the replacement rear speaker, I called S&M and they spent a fair amount of time on the phone walking me through the various options from 1967. They're a bit like historians concerning the U.S. automakers and had some interesting bits of info about the options available across the GM lines and model years.
They should easily be able to walk you through whether or not you can use your existing single line feed/speaker switch and how to establish the correct ground. As JMT mentioned, the ground wire loop and crimp fittings came in my box and they package the speakers according to your specific application.
'67 GTO/Signet Gold over Parchment/Pontiac Built