Taillight Wire - Spot Lights

[farm1810]

I have looked at the threads about wiring in spot lights / running lights. I will be using a relay, mounting it under the seat, fused, etc. I am thinking that the blue wire for the taillight should be in a harnass running under the seat. Would it be sufficient to power my relay from wire running to the taillight so the spot lights are only on when the key is turned on? I am also going to wire in a manual switch up front. Will this wire / circuit handle the load of the relay? Thanks. R.