Have you just looked for the specs on the GPS unit? I have several Garmin products. They have their specs on-line...
You could just get a female cigar lighter socket and wire it to an DC supply that puts out 11V to about 16V at 1A or more. I have several such DC supplies that I bought at garage sales that formerly powered Laptops, Printers, Modem, etc. Most of the 12V DC plug-in adapters contain an IC chip that is a switch-mode step-down voltage converter, and they have a wide tolerance of input voltages...