There is 3-prong jack on the back of the radio. A 2-wire connector from the front speaker plugs into the radio first. The dark green wire from the speaker ties to the center pin. The black wire from the speaker ties to the right (passenger-side) pin. The diagram does not show the black wire to be grounded at the speaker frame.
Then the yellow power feed wire plugs into the left (driver-side) pin on the back of the speaker connector. This arrangement makes sure the speaker is connected before power can be applied to the radio. Radio damage can occur by running it without a speaker.
A separate dark blue pigtail wire leading off to the right from the radio is the feed for the rear speaker, if used. The other terminal of the rear speaker is grounded at the speaker frame.
The right-side outer ring fader knob controls the front-to-back speaker volume where the rear speaker is supplied. This knob along with the attached resistor assembly should be in place before powering up the radio, again to prevent radio damage.
If no rear speaker is used, the right outer ring knob is a dummy, and a jumper is installed across two of the pins on the radio case where the fader control would go.
The 68 Buick dealer sales brochure gives a good run down of all the standard and optional equipment. These are available regularly on ebay for reasonable prices or you can have a scanned copy of that part of mine. Although it is not an official Buick publication, I also have an option price list for 68 Buicks available. If I dig it out, I have a copy of the radio schematic diagram as well if you need it.
