Pete,
First, does the flood light have its own built in switch? If not you will need to find one that you can install to toggle the light on and off. Second I would also buy an inline fuse to protect the wiring for the spotlight. The best place to connect under the dash would be terminal 1 on the light switch, ( same lug for power to the horn) or on the Non Battery side of the ammeter. See the attached wiring diagram.
Good Luck,
Glenn Manes
Wheat Ridge CO, USA