I've decided to revive this thread in case anyone is still having problems. By disconnecting one wire from the GEM under the dash, I was able to solve the parasitic draw problem in my mustang without any adverse effects. Someone with an official wiring diagram can help to complete this post.

I have a 2003 V6 coupe with ABS and traction control and for the past three years I have been wrestling with battery issues. I replaced my alternator last year but no avail. Whenever my car sat for more than a week, the battery would be less than 10 volts and be unable to start the car. I've gone through 3 Everstart batteries – each one lasting <12 months – and whenever I exchange them Walmart says it had a bad cell. I started to question the probability of this happening to every battery I exchanged so I turned my attention to my car's electrical system.

Batteries dying seems like a common problem for all of the 94-04 mustangs. Some people's solution is to take out their CDs and turn off the radio; unfortunately, this did not work for me. Others suggest using a trickle charger whenever the car sits. However, I REFUSE to cover up this problem by putting my car on trickle charger. If these cars can not sit for longer than a week without dying then they are HORRIBLY engineered vehicles.

I removed my negative terminal and connected a multimeter in series (one probe on the negative terminal and the other on the negative lead) to measure the current. After hitting the lock button on my key fob to turn off the interior lights, the current drops to 150 mA. Pulling fuses one by one, I narrow the problem down to the GEM (fuse #39) as the only circuit drawing power. I put the fuse back in and wait several hours to see if the computer goes to sleep, but it never does. Doing a simple calculation for a 150 mA draw with the stock 40 Amp-hour battery gives: 40/.15/24=11 days. It's no wonder my battery only lasts a week.

While pulling fuse #39 stops the parasitic draw, it is NOT the source of the problem. For some reason the Powertrain Control Module (PCM) is not putting the car to sleep after it sits for a little bit like it is supposed to. I start disconnecting wire harness connectors under the dash to find the signal that is keeping the computer awake. I discovered that if I remove the black connector closest to the door under the GEM behind the driver's kick-panel, then my 150mA current drops down to 100mA. What's great though, is that after 40 minutes, it goes to sleep and drops to <5mA. I measured all of the voltages of the wires in this plug and discovered that the black/white wire is the only one with +12 VDC. This wire is actually connected to the interior lighting fuse, #7, and not #39. I removed the black/white wire from the black connector and plug the connector back into the module. The current went back up to 150mA, however, the computer still went to sleep after 40 minutes.

Everything seems to be working just fine and the remote keyless entry, interior lights, wipers, warning lights, power windows, one-touch driver's window, and heated rear window still work the way they are supposed to. The GEM has other +12 VDC wires powering it so I don't really know what this specific wire is actually doing other than playing havoc with my computer's sleep function. Anyone with an official shop manual or detailed wiring diagrams for the 99-04 V6 models may be able to figure out exactly what it is. If this wire is not the actual problem, then I would like to narrow it down even further to figure out which component is faulty or where there is a short.

The wires in this black 12-pin connector for the GEM are as follows:

brown/red____________yellow/white
gray/blue_____________(empty)
purple/yellow__________(empty)
black/white(removed)___(empty)
(empty)_______________black/blue
black/purple___________(empty)