Aftermarket Alarm Double-Lock aka ANY Alarm Can Double-Lock
BMW doors have a two stage lock system, you can either "single-lock" or "double-lock" them. "Single-locking" only disables the exterior door handles and does not lock the fuel door (may vary by year). "Double-locking" disables the exterior door handles, locks the fuel door, disables the interior door pulls and (rumor has it) disables the starter. When the doors are "double-locked" even if someone breaks a window, they can not open the doors or trunk by pulling an interior door pull.

The OEM add-on alarm/keyless entry systems have "double-locking" built in to them. However lots of folks would rather use aftermarket alarm systems to save money or because of the added features available with non-OEM systems. The trick is wiring up the aftermarket system to "double-lock" the doors for added security. "Double-locking" is activated by sending a 12v+ pulse to both the lock and unlock wire on car's central locking system simultaneously. It's really simple once you know what to do.

You will need the following parts/tools:
- Install manual for your alarm
- 2 x Diode (1A) from Radio Shack
- 18ga Wire
- Solder
- Soldering Iron
- Wire stripers
- Electrical tape
- Heat shrink

This "how-to" will assume that either you already know how to wire up your alarm and/or you already have your alarm installed. It will only focus on changing your aftermarket alarm from "single-locking" to "double-locking".

Locate the LOCK and UNLOCK wires on your alarm system's brain. Cut the UNLOCK wire, strip the ends of the wire and slip a piece of heatshrink over both ends. Solder in a diode that will only allow current to flow out of the alarm (striped end facing away). Once the solder joints have cooled, slide the heatshrink over the exposed wire/diode connection points and heat the heatshrink. Solder 3" lengths of wire to both sides of the other diode, use heatshrink to cover the exposed solder joints and then strip ~1/2" of the ends of the wires. Use wire strippers to pull back the insulation from any point on the LOCK wire and the UNLOCK wire somewhere beyond the first diode. Tightly wrap the ends of the wires, soldered to the second diode, around the exposed wire on the UNLOCK and LOCK wires with the striped end of the diode facing the UNLOCK wire. Solder the wrapped wires and use electrical tape to cover the joints.

Test your "double-locking": roll down a window and then arm your alarm. First try to open the door using the exterior door handle. If you can open the door then you've wired the first diode in the wrong direction. If not, then reach through the open window and try to open the door using the interior door pull. If the interior door pull works then you've wired the second diode backwards.

WARNING: If your alarm is set up to lock the doors when it passively arms (auto-arm) then you need to be careful. If you're sitting in your car without your keys and your alarm passively arms itself and "double-locks" the doors, you could be locked in your car with no way to get out! How embarrassing would THAT be? ;)