problem with my aftermarket radio

If it only has 2 places to plug the leads into, it should be obvious for you.  If it has more than 2 spots for the leads, one is Probably labeled V - Ohms, this is the one you want to plug the red lead into.  The black lead plugs into the one labeled Common.  Set it to the DC Volts setting.  If it has multiple ranges within the DC Volts section, set it to the 20 Volts scale.  If it has 2 Voltage settings but does not signify AC or DC, set it to the one with the Straight line and the dashed line.  That is DC.  The AC Volts setting is signified by a WaveForm.   With the meter set to the proper setting, touch and hold the black meter lead to the Chassis of the radio.  Now touch the red lead to the Yellow wire of the harness that plugs into the original radio harness of the vehicle.  From what you are saying this is probably not going to show 12 volts.  If it does not, get your owners manual and see if you can find the location of the fuse.  There are 2 fuses that power the radio.  There may be only 1 labeled radio.  The other one may be labeled lighter, courtesy dome.  If you can not locate it with the owners manual, turn the key on , turn the parking lights on, touch and hold the black lead to the chassis of the vehicle.   On every fuse in the fuse box there are 2 contact areas the you can touch the meter lead to.  Touch the red lead to one contact of that fuse, notice the reading, then check the other contact.  If you find a fuse that reads 12 V on one contact and has no 12 v on the other, pull it out and inspect it.  Make sure you notice the location before you pull it out.  There are probably more than a few empty locations in the fusebox.