Do you need access to the brake light wiring? If so, you'll need to access the wires at the tail light.
There is a hole in the rear fender where the tab from the tag/turns assembly goes into the rear fender. You'll have to unbolt that assembly from the rear fender. You can run your wires through there up inside the fender to the tail light to access brake light wires. You can splice into the wiring there.
http://www.hondadirectline.com/OEMpartfinder.htm#/Honda/GL1500C_A_(97)_MOTORCYCLE,_USA,_VIN%23_1HFSC340-VA000001/REAR_FENDER_+_GRAB_RAIL/GL1500C-97-A/2Y14MZ0VMZ04F2600AEhttp://www.hondadirectline.com/OEMpartfinder.htm#/Honda/GL1500C_A_(97)_MOTORCYCLE,_USA,_VIN%23_1HFSC340-VA000001/TAILLIGHT/GL1500C-97-A/2Y14MZ0VMZ04F2800ADIf you don't have one already, get a manual.
Honestly, for me, while doing any wiring on the rear of the bike, it was way easier just to unplug and remove the entire rear section of the rear fender and put it up on the work bench/table to get access to what I needed....however, it can be done easily enough on the bike.
My tag frame is from Custom Dynamics...it's a run/brake led (red leds with clear lenses) on the upper part and on the lower part of the frame. Very bright. That's how I wired mine.
The red reflectors on there the same way....they're not just reflectors...they'er running light leds also.