I weighed all the options when I was getting ready to hook up my CS spoiler as well. The easiest thing to do is to tie them into the side markers, you should have enough wire without having to add any. I just spliced the wires together and never had any issues what so ever. I was able to run the CS fog lights when I turned on the parking lights, then when I pulled the switch out the fog lights in the grill came on also... it was actually pretty cool running the 4 fog lights at once. Good luck, you're going to love the look!!!!
2014 Roush RS3