Just curious...do these things have anything in them? Since you're going from 12VDC to 12VDC, I wonder if some manufacturers took the cheap route and built totally passive unfused cables. Also, if it is passive and ends up working well...it would be dead simple to hack together your own provided that you had spare cable ends.
Not on this one, at least. It seems to have a 5 amp fuse, but that might be all in it. It also has a vent on it, so something has to be in there.
It's most likely just a passthrough. I looked to see if other 12V adapters did this and apparently they do. Every car adapter I've seen has a fuse in it as well. Pretty standard. http://www.pocketgpsworld.com/seidio-g2500.php