I have a '05 Mustang, the battery keeps draining when the car is off, I have replaced the alternator, the battery and still no fix, I also rewired the charging system when I relocated the battery to the trunk, this problem started before this, I have an amplifier, but it works off the signal of the cd player not a shaker either, the battery drained from 12 to 6 volts in 24hrs. On a side note, my horn stopped working a while back, and keylessentry for the trunk doesn't work either. Something has to be using voltage when the car is not in use, I just can't figure it out.

 

Get yourself a meter with a clamp-on current probe. Attach the probe to observe the drain on the battery with the car off, then start pulling fuses one at a time until you find the circuit that’s pulling the most amps. The computer and clock will use a tiny amount btw, but not enough to kill your battery. I’d recommend to start with the stereo equipment or anything else not factory.

 

I started doing that last night, I have a fluke, only bad thing it doesn't meter after the decimal place, but I pulled some fuses and relays, I pulled the relay for the horn and defroster, and pulled fuses for the pass. window motor, radio, trunklid release and I disconnected the amp too. Something I noticed last night when I pushed the button for the window the lights would dim slightly and a clicking noise would come from passenger side door. The car started without problems this morning, so hope
fully I'm on to something.

 

Are we talking it drains after being parked for 2 weeks or it drains after being parked for a day?

If after 2 weeks, this is a common problem. The car is not "off" when the car is "off". Security system, NAV unit, Radio, etc may be the cause since they are drawing some current when the car is parked (not sure if anyone ever really figured it out). During the first year, my car battery was draining if I parked it for about 2 weeks or so. Since then, if I know my car isn't going to be used on a given week, I hook it up to a trickle charger. Many threads on the issue. Here are a few:

https://themustangsource.com/f739/ba...roblem-479609/

https://themustangsource.com/f739/dead-battery-427426/

https://themustangsource.com/f739/de...lready-446863/

 

It's over night, I know some ford's can draw about 300-400 milliamps while the car is off, if it was over a few weeks I wouldn't be on here, but not when this problem has cost an alternator and 2 batteries.

 

I was in ur same shoes, 05 mustang, amp/subs, and my battery started draining and my car died sometimes. The first time it drained on me the charging system checklight came on, traction control turned off and the came went completely dead except the engine was running. I took it to sears (where I bought my battery) and they charged the battery, checked the alternator and everything was fine. After about two months the battery slowly started draining again and the car died so I took it to sears and they recharged the battery and said they couldn't find what was wrong. I decided to take it to my uncles shop and after some time he identified that there was a loose cable to the charging system which was causing all my problems. It was a simple fix, might be the case for you.

Edit-The problem came after my header install so I think I might have knocked something loose during the install.