While you are at it, you might consider tapping into the tail light too. I have a handy little diagram and some photos for a mod to the honda led brake light which allows it to be seen as a tail / stop light - operating at reduced brightness all of the time, but full brightness when brakes are applied. I assume the givi has LED lights ?
I've done this twice. First time i cut all 3 wires and fitted a hitachi 3 pin connector to remake the connection, with an extra set of leads running up into the rear cowl under the seat. The bike was 6 years old, and the cable inside was turning dark. I felt i had to clean the strands in order to make a decent connection. 2nd time, i didnt cut the wires. I 'peeled' the plastic insulation like peeling a potato - very sharp scalpel, slicing parallel to the wire. I twisted the extra wire into it and soldered, greased the bare wire and insulated it. Impossible to get sleeving on doing it like this, but lay the joint along a length of insulation tape and fold the sides up so that the exposed sticky sides can be pressed together. That seals the grease in, then wrap round with tape in the normal way.
While you are in there, disconnect the connector from the bulb, and apply some dilectric grease. On my older bike these were very weathered and were difficult to clean up. Unlike most haonda connectors, these had been left unprotected.
[Edit] Just realised I didn't answer the original question
To get at the tail light leads, remove the rear mudflap (4 bolts). The bulb holders turn and pop out. Best to undo both, and remove the bulbs to save breaking them. It is probably easier to tap into the lead from the harness which feeds to the right hand bulb. There isn't a great deal of space to work, especially if you are trying to crimp connectors onto the leads.
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I'll start another post re the tail stop light, and put the link here.
Here it is.
