Nokia speaks? damn. The speakers in the rear deck are size 4T. A few companies make these, including Pioneer if I remember right. If you want to put 6x9s in the rear deck and want an easy way out check these out.
http://scorpion.makes.it
As far as testing if the front speaks work, you could pull the deck and hook a speaker up directly to the deck. If no sound then there could be a problem with the wiring connections or factory amp if you have one. You could also buy a multimeter and test for voltage through the wires at the speaks.
Hope this helps.