My fiance's 2001 V6 failed to start this morning while I was on my way to school. It did this a few nights ago, but before I could get my tools out to test it, the thing started behaving itself. Now it's failed again, and I'm not around again to check it. Here's what I know

1. The door ringer, dome light, power locks and windows, everything works except the starter.
2. She mentioned hearing a clicking sound coming from the ignition WHEN THE KEY WAS OUT of the ignition. That's puzzling. Said it's a constant sound, not extremely rapid, about 1/2 second to 1 second interval.
3. I don't know the condition of the battery, that's my first thing to check.
4. 133K miles on the car.

Assuming I don't find a dead/near dead battery in the car, what about the starter/relay? How bad are they to change and is the relay integrated into the starter? What about the clicking coming from the igntion? Any advice would be welcome, I have to get this thing fixed tonight if at all possible. I'm new to Mustangs but I always do my own work on my cars.