Originally Posted by NicholasP
Well I wired the under glow to the dome light but now I can't get the switch to work its just a loose button so now I can't turn on the under glow when driving haha
The Mustang is rejecting the underglow. Listen to it.
When you wired it to the switch, did you get the correct lead? Did you wire the ground circuit of your underglow to the switch? A switch normally opens/closes the ground of a circuit; rarely the power, to prevent arching.
Check your fuse too.