Installing logo projector

Does your projector draw more than 5 amps current? Then it might be too much for the back light circuit. Pick a stronger circuit like head lamp and, first thing, put in an inline fuseholder for a 10 amp fuse. Then wire to the switch and a 1-wire back to the projector. The projector ground wire needs a solid connection to the metal frame.
If you take the power from the back light, first use a fuseholder. Run a sturdy 2-wire up to the switch up front. The switch return goes to the projector and then to ground.
To fish the wire under the tank I use an old long flexible transmission dipstick with the handle cut off. Feed the dipstick from back to front until it comes out where you want it. Tape the wire to it and pull it through. Be sure the wire has no pinch points. Secure with tie straps as much as you can. Feed the front end of the wire up to the switch. Feed the back end to your device.