There are two ways to go about providing a 6v power wire to the clock. One way is to go to Radio Shack and get a cigar lighter power cord that has a switchable output. Set it to 6 volts, wire it in and hide it under the dash.

The other way is cheaper and, to my way of thinking, a bit more elegant. Get a 7806 voltage regulator from Radio Shack and wire it in. Here is a web page to explain how:
http://www.gsdi.org:8088/Tech/6-12v/6-12-6.html

Your clock probably won't pull much current. Low current draw means less heat. You can probably just mount the regulator to any convenient metal surface for a heat sink. Do use some heat sink grease, though.
Hope that helps.

Regards,
Shannon DeWolfe