I've read where someone suggests tying into the dash display light. Good luck, it's a giant bundle of wires that vanishes into the display, no user serviceable parts inside.
It's easy to make a light come on only when you are in reverse though, and you don't even need to run any wires up to the front dash.

Just wire them like a brake light, with your switched hot (always on when the key's on) to the light's hot side and splice into the reverse wire on the gear position sensor for the negative wire.
(All that 'sensor' is, is a rotary switch on the end of the gear selector drum that applies ground to whatever wire is associated with the gear you are in, including park.)　
The gear position wire for reverse is sky blue and white.  You can borrow switched hot from the brown wire in that trailer plug under the left rear fender or always hot from the handy starter solenoid (insert an inline fuse holder and fuse!).
A handy place to find that grounding wire is on the left side just ahead of and below the CVT cover.  I cut the shield lengthways with a scalpel and dug out the blue wire. Ordinarily I abhor scotch locks but this was a one hander way in there so I took a lazy way out.
Cheers.

https://imgur.com/KpDoJnY