The cigar lighter is usually rated at 10amp as it is a heavy load. Your map light would not be anything like that and if it is an LED it would be in micro amps.
You could run a separate fuse but the existing fuse would be fine.
Titus

Titus

2007 +4 4 seater
2017 Mini Cooper
2007 VW Camper
1996 Honda TRX300
2012 Husqvarna 216