[Richard]

If I read your correctly, all five speakers will work if hooked to one specific output on the Yamaha. But the other outputs on the Yamaha do not provide sound. Do all five (or four) channels work when you are using your dash radio?

Are the relays powered by the dash or the Yamaha?

But it does sound like only one channel of the Yamaha is putting out sound. Please ensure that the balance and fade adjustments are center, and you haven't accidentally adjusted the receiver to bias all the sound into that channel.

You could of course hook up any old speaker or earbuds to the other channels to check too.