Won't start

I have a 2003 Ford Mustang it has been running fine, then just today went to try to start it and there was nothing. I thought it might be the starter but I took the starter to a auto parts store to have it tested and there was noting wrong with it. I then purchased a starter relay hoping that might be the problem, no luck. The car has juice its just not sending anything to the starter, can not figure out what the problem could be. Please help.