As Bernard suggests, you do need a correctly rated extra inline fuse so as to protect yourself from losing the tail light if your grips/gps malfunctions whilst on the move.

Also remember the supply will be live when the ignition is in the parking light position with the key removed, so there is potential to flatten your battery in double quick time if you forget and leave the grips switched on and do this. I know a fair few people (including me) who have walked off and left their bike in daylight without realising they'd turned the key round one notch too far - easily done!