My 2005 V6 mustang battery is going dead after sitting for about a week. I put a new battery on it. Something is draining the battery. I heard what sounded like a relay clicking under the hood in the general area of the fuse panel.  

I don't know if the new Mustangs have a glove box or trunk light, but check to make sure those are not staying on (misadjusted latch, loose light housing, etc.

Otherwise you'll have to start pulling fuses to see where the draw is...

BTW, you might want to visit our sister magazine web sites where they focus more on the new Mustangs:

www.50mustangandsuperfords.com
www.musclemustangfastfords.com

HTH...
Mark  

Thanks! I've learned that the "SHAKER" radios are having issues with this if not turned off before turning the ignition off.  

Good to know, and thanks for replying back. If someone has a similar issue they'll see your reply in a future search, and that'll help others.

Mark  

I can help to confirm this, I ride a motorcycle so my 2006 GT sometime sits for a week unused, I had two dead battery events in the last few months. Then I read about the Shaker 500, so this holiday I turned off the radio before shutting down the car and it was 100% OK after 7 days standing.  

I just had a similar occurance in my 2006 V-6 convertible The dealer told me it was the presets in the radio draining  mili-amps, but over time it can be cumulative. So now I guess I need to remember to turn off my Shaker 500 stereo before turning off the car!  

Here's a similar thread - points to a potential problem with the fog light relay as well as the radio.  
Link to dead battery Thread  Google on "2005 Mustang Dead Battery" and you'll come up with 25000+ hits.  

I have had this problem with a couple of cars, most recent my 2001 Mustang GT.  Turned out the regulator is bad in the alternator.  You have to replace the whole alternator to fix the problem.  Also had the same problem on an 86 Couger, but the regulator was separate, easier fix.