What I gather from the instructions you quoted is you need a constant 12 VDC source near your slideout switch. Is there a light or any other hot near the switch/, if so run a wire to an inline fuse holder to the center post of the switch.
The instructions did not say anything about running a new hot wire to the relay. You have two wires that run to the relay already, that is all you need.
If you are having trouble understanding the instructions, PM me and I will make a drawing and post it in your original post.