Unfortunately, you’ll need to spend some more money on an amplifier for the deck speakers if you want to do this right. Theoretically you [i[could[/i] use a simple speaker selector switch connected to the Yamaha’s “B” terminals that would let you use the porch or deck speakers separately, or both at the same time. However, with that you’d lose independent volume control of the deck.
For instance, if the speakers you choose for the deck end up being less efficient than your other ones, or if you have some signal loss due to the long speaker cable run, you might end up with your deck volume being lower than you want. A local volume control (between the speaker selector and the speakers) would allow you to reduce the volume, but it won’t boost the signal if it’s too low.
I’d suggest a simple stereo receiver or integrated amplifier, fed by to the Yamaha’s Zone 2 RCA outputs. I’m sure you can find something cheap at a pawn shop or on eBay. That would give you an independent level control of the deck speakers. You could also wire in an L-pad at the deck, to give local volume control w/o having to run all the way back to the house.
For speakerwiring, you’ll want to use cable that’s rated for direct burial. Some 14-gauge wiring for low-voltage lighting will do the trick. It looks like standard zip cord, only it’s black. If you can't find any in the length you need and have to do a splice, use waterproof splices rated for direct burial as well. You can probably find those on the electrical aisle at the hardware store.
Regards,
Wayne A. Pflughaupt