I run my GPS power directly to the battery through a 5A in line fuse located at the battery. A fuse block is the way to go if you plan on running other circuits but just for the GPS I would go this way. I like having the unit run off of the bike when I power down for gas and don't have to worry about restarting when I ride off. I haven't left it on yet but that might be an issue if you have those kind of problems.
Although my little el-cheapo Garmin is not
waterproof, a zip loc bag suffices nicely.
I also like to leave the GPS powered up
when I turn the key off to refuel, and to
recharge the internal battery after I arrive.
Although I intend to install a 12v. socket
(probably a waterproof one) into the bezel
beside the speedo, I prefer a dedicated
GPS power plug. I use a small rubber cap
to insulate the power plug when not in use.
I bought a simple five-fuse block at O-Reilly's
to install under the seat for multiple circuits.
It will feed off a 20 or 30A. breaker and
incorporate a Battery Tender pigtail for power
during maintenance and testing.
Unless you live in AZ or other dry, desert
areas, best to use NOALOX (professional)
or 'Bulb grease' on all connections. Solder,
heat shrink tubing and inexpensive split wire
loom can combine for a real professional job.