2008 vw rabbit factory momentary ground

So basically, I have an old alarm with a remote that I want to add to my car so that I can use the siren and shock sensor without extensive modding. I want to solder the arm/disarm leads of my aftermarket alarm remote so and connect it in the car (leave the aftermarked remote stowed away somewhere in the car) then connect it so that when i lock the car with the factory remote, the arm "button" on the aftermarket is triggered, thus arming the alarm, disarm=the same thing. it makes sense and would be convenient. Here's what I need to know...

there are two pins in the aftermarket remote under the ARM/DISARM buttons. When you push ARM. the two pins touch... What I want to do it find a ground source that only gets ground in the car for a second or so when you use the factory remote LOCK/UNLOCK buttons. When I find that ground signal souce, I can solder it to one of the pins in the aftermarket remote and the other to a constant ground.... There is no load since the circuit would only be closing ground. If there is a positive source, I would use a relay accordingly so that there is no additional drain....

I would think your best bet would be to use the door lock actuators.  The major drawback here is that if you reach through an open window and hit the unlock button on the door it will disable the added alarm.

If your car has priority unlock you could possibly set something up that would look to see if ONLY the drivers door is unlocked send the momentary signal but if the passenger door and the drivers door are unlocked don't send the signal.

You would definately need relays to convert the + to the negative you think you need.  Keep in mind though, that unless you connect the remotes ground to the cars ground sending a (-) from the car to the remote won't do anything - the grounds have to be common in order for that to work.

If it were me, I would make a small 3vdc (or whatever voltage the remote works at) power supply and hardwire the remote to the car battery (a dead remote battery would be a pain).  I would then use the contacts on the relay to push the button by shorting out one pin to the other.  Use the relay contacts as contacts instead of using them to send a (-) or (+) signal.

The question is though does that signal change when you remotely arm and disarm the car?  Typically, those signals arn't activated by the remote, but by the key in the door cylinder or some other method.

If that signal does change when you remotely arm and disarm the car then you could use it for what you are trying to do.  You'll just need to build a simply interface circuit to look for the correct voltage.