My last two bikes I put the cig lighter at the front of the bike so I could easily remove the gps power cord (saved buying a second one to use occasionally on the bike).
My last bike I was able to zip tie it to the ignition with good clearance. The cig lighter came with a rubber top and the rest was sealed so not a problem.
I've seen other guys just have a wire poking through for power. If it has a weatherproof end that's great, if it doesn't then at the least wrap it in some electrical tape when not being used to keep out water and dirt.
And easier way to mount a cit lighter is adhessive-backed velcrow to the bike and cig-lighter. There are tons of options out there, the only concern is that it can stand up to the elements, the wire has clearance with the handle bars turned fully left & right, and that it won't come loose from vibration or a big pot hole.
My current bike the ideal spot was to bolt it to the back of the headlight housing, using some screws with a bit of loctite. For my bike I checked and found that portions of the headlight assembly are hollow (makes sense, square housing, curved reflective headlights spaces).
I ran the wiring from the front of the bike to the back following how the main electrical wire trunk flowed through the bike. Following the trunk is a safer assumption when it comes to clearance and engine heat instead of doing the shortest route through the bike.
The last bike had 10,000km+ of gps power with no issues, the current bike has 9,000km+ with no issues.