You'll need switch, inline fuse and a relay to connect everything up (with LEDs you could connect directly to the lighting circuit), but I'd go for a separate circuit.

Splice into the tail lamp supply under the seat for the relay trigger, main switched supply direct of battery (fused), relay goes under seat and switch on bars...

Here's a diagram that might help?





If you don't fancy having a go there are plenty of people who do made up looms like Adventure Spec and GULG...

...or (as your not too far from me) I'll could give you a hand to make a custom loom on your bike?