Assuming the multimeter is like the one in the picture below set it up like this.
Not knowing how it is wired, have the car key turned to accessories and open the fuse holder and connect the red multimeter cable probe to the battery side and the black cable to an earth and it should show around 12V on the multimeter.
If there is voltage there make sure the fuse is ok, if you have a spare the same then replace it anyway and use the multimeter to check for voltage at the red wire of the power plug on the radio with the black lead on earth.
If you would prefer I talked you through it on the phone let me know and either you pm your number or I could pm you mine.
Multimeter.jpg