I am thinking about adding an older Garmin Street Pilot GPS. I will need to buy a new 12V power adapter and can get them with the cigarette lighter end installed or with just a two wire end without the cigarette lighter adapter.

Any suggestions about the best way to go? It seems maybe just a small pin and plug would handle the load and be less clunky. Keep in mind that I know nothing about wiring so any specific parts, pictures, or schematics would really help.

Thanks in advance

Most mobile GPS units can actually run from batteries, so the power requirement is small, generally on the order of a few 100ma.   One clean way of doing a power hookup is to use a simple 3.5mm phono jack & plug.   Note that you only need the 2 conductor type (hot tip and ground), not the 3 conductor stereo type.   If you buy a panel mount jack, you might find a place to drill a hole and mount it on the plastic speedo housing.   Radio Shack carries them or these would suffice just fine...

http://www.allelectronics.com/make-a-store/item/MMP/MONO-METAL-3.5-MM-PLUG/1....

http://www.allelectronics.com/make-a-store/item/MMJ/3.5MM-MONO-OPEN-AUDIO-PHO...

Anytime you wire directly to battery power, it's a good idea to fuse the +12V line with an inline fuse (also at RS or online).   For the GPS a 1 or 2 amp fuse is fine, or use a bigger one if you plan to use other loads (accessories).   The idea is in the even of a short, the fuse blows instead of the wire glowing red hot and melting itself and possibly other stuff.

If it was just a light I'd say the chassis ground would be OK, but the GPS may have noise sensitivity so it's probably best to go straight to the terminals.  Also once you install the 12V jack, you may plug other unknown stuff into it in the future.  Be sure to place the fuse close (electrically) to the +12V battery terminal.  That way, anything that causes it to short (rubbing thru insulation, etc.) after the fuse, will blow it and minimize any danger of fire.   Fires around the gas tank at 60mph are not good; in fact, fires around gas tank at 0mph are not good.