Proper (and best) setup:
Get a separate amplifier.
Connect your tape output (recording output) from the receiver to this amplifer. It doesn't have to be a great one, but it will ensure that you are taking the load off the receiver and putting it on the speakers you are using outside.
Run it from the amplifier output, to a volume control in your garage.
Then run from the volume control to your speakers.
When you want to use speakers in your main room - do so as you normally would, just turn on the receiver, and use your system.
When you want the speakers on in the garage, turn the amplifier on, or turn the volume up on the volume control outside, with the receiver on.
Always remember, if you aren't using a receiver with a true 'zone 2' output, you will be listening to the same thing in both areas as you only have one single 'zone' of audio.
Don't mess around with trying to connect a bunch of speakers to the amplifier inside the receiver as this is a sure fire way to end up needing to buy a new receiver.
An amplifier like this will do the trick nicely:
http://cgi.ebay.com/AudioSource-2-Channel-100-Watt-Power-Amplifier-AMP-100_W0QQitemZ110226071918QQihZ001QQcategoryZ14973QQssPageNameZWDVWQQrdZ1QQcmdZViewItem