There should be a main power feed, maybe a separate dial light feed and a feed for the speaker. If you hook up your ohm meter between ground and each wire one at a time, you should be able to tell which ones are for power. Turning the off/on to off will give you a very high resistance and the on position will show a low resistance.
I haven't used those aftermarket speakers but I would say hook the speaker feed to one terminal of each speaker, in parallel, and then connect the other speaker terminals to ground.
Here is the corrected link to your radio photo that you posted.
radio photo 
