Constant 12 volts for new radio

I'm putting a new radio in my '77.  It requires 2 sources of power, one constant 12 volts for the clock and one "accessories" which is on when the ignition in on.  The three-wire plug into the original radio has a black (ground), yellow (accessories), and a gray, which is not indicating any voltage.  Should that be the constant 12 volts?  If so, any help on why it is not hot.  If I have to get constant 12 volts from some other splice, please guide me to the proper wire (color) and where it is located.  Thanks!

The gray wire is for the light inside the radio at night. Turn on your lights and check the wire for voltage after that. The cigar lighter/clock would have a constant 12 volts.

The Gray should be for lights for the old radio. turn your headlight sw on and you should see 12 v then.

Hi Dan,  The color of the constant 12v wire to the cig lighter is Orange. If you choose to run a new wire from the battery positive terminal, I suggest that you use an inline fuse (10 amp)close to the battery connection. This fuse will protect the wire from meltdown should it short to ground.

I'll second Joe's comment - inline fuse close to battery is definitely a good thing.  A 12 ga wire directly connected between an almost infinite current source (e.g. a car battery) and ground will get very hot very quickly.  I have some small jumper wires for electronics (used them to bench test my clock) that have the insulation burned off and they were only inadvertently connected for a second or less!  

I simply tapped into the cig/lighter (orange) wire.  This way, no need to run a wire under the carpet and no need for in-line fuse.

I found that the positive post on the clock is right there on the back of the instrument cluster.  I merely tapped into the 12v supply with a round connector and about 12 inches of wire with a lag connector to the wiring harness for the new radio.