Before doing this, be aware that you are dealing with a potentially dangerous amount of current, in light of this do not touch any diametric wires together as a spark will most likely be emitted which can easily start a fire. 

This has two very useful purposes: 1) It can possibly provide a cheaper way of getting audio into your room and 2) Testing a head unit to be sure it works, and possibly even taking pics to post on ebay (I've found my head units sell better when the consumer sees that it is on and working).

What you'll need is basically a high current power supply(15 amperes should be fine) that puts out

about 12-14v DC and car speakers with appropriate boxes plus wiring. Astron and Tenma are my personal favorite and can be had on ebay for excellent prices.

Hook up the yellow battery positive(+) of the head unit to the positive (+) terminal of the power supply and the black ground(-) of the head unit to the negative(-) terminal of the power supply. Hook up the red ignition (+) of the head unit to the (+) of the power supply, but be sure there is a switch there to turn it on and off as a car does (if you leave the power supply on, since constant power is being provided to the battery positive, you won't lose your settings). Then hook up the speaker wires as you would in a car. Not too difficult! :)