Retrofitting Wonderbar into non-wonderbar '66

[Twilight Fenrir]

Hey... what's that third connector on the speaker output do? My car has a front, and rear speaker, but there's no third wire on the harness. It's obviously for something the Toro does not utilize... but, what exactly?

Also, looks like I'm going to be putting in an amp, so I can use some modern speakers. Probably a little overkill for such an old radio, esp. when I won't even be getting stereo out of it. But, I want the best sound I can get out of it. No way I'm going away from the factory looking radio. (Might consider one of those digital rebuilds sometime in the future)

[69W34]

Only the General knows for sure and ain't life just full of surprises