glove compartment is the best place to install a cig lighter plug.
because:
1- right next to the contact/ignition switch, where you can directly tap off the correct wire
2- Dry
3- Not immediately visible, so you won't see it easily if you did a lousy job at drilling the plastic
4- Close by dash in case you want to install a GPS and don't have a long USB cable with you
5- In case you do have long wiring, you can keep most wiring within the glove box, keeping your dash less cluttered with cables while riding
6- Quite nearby your head, in case you want to plug in an MP3 player
7- enough material to drill another hole in case you want to expand or try again :-)