If you really want a 'stereo' with only a 12v and earth connectors then you'll need something off of the '70s with a built in speaker. Like this.

If however you want to make use of the speakers you've put in the parcel shelf, then you'll need another pair of wires for each channel(side).
Assuming you're using a head unit(bit in the dashboard with the Knobs on), which was built in the last ten years, then it will come with a universal multi plug on the back, and you'll need a thing like this to wire it up.
http://cgi.ebay.co.uk/CAR-STEREO-MALE-I ... 450wt_1199They're also available in any halfrauds type place, or by cutting one out of any modern car in the local scrapyard.
They're not as scary as they look, you don't need to use all the wires, just the ones you want