shock sensor to porsche fact alarm?

Hello electric gurus,
I own a 1997 Porsche 993 with factory alarm system and immo.
The problem is, I want the shock sensor to switch on and off with the alarm system. It's a DEI 504.
It uses 12V supply voltage and has a 12v pos. input switched.
I need to find a 12V pos. source to feed the power to the shock sensor, only when the factory alarm is triggered with the key fob. I have schematics of the alarm system but there is no 12V anywhere when the alarm is on and no 12V when the alarm is off.
There are 2 wires to the alarm ECU for on and off. I thought I could hook up a relay with 2 coils or a NAND logic gatter but no worky. Upon measuring with the multitester, both wires are at 8.5V. When I activate the key fob, one wires goes momentarily to 4.2 and right back up to 8.5 while the other one remains at 8.5. When pushing the key fob again. the other contact will momentarily go from 8.5 to 4.2.
What a weird circuitry.
Then I thought about using a time adjustable delay to KL15+, the switched plus in the car. Once I kill the ignition, I would have x amount of time to supply power to the shock sensor. That would work getting out of the car but coming back would trigger it again because I can't turn on the ignition w/o triggering the sensor.
Sorry for the long post but at this point, I'm clueless on what to do next. I checked every cable that goes to the alarm ECU and none can be used to switch power off from.
I'd appreciate any help.
Thank you,
Ed

Possibly a magnet on the door lock rod and a reed switch mounted to the door positioned so the magnet trips the reed when the doors are locked. Does the factory alarm have a security light that lights up when armed? Or a flashing LED when armed?

Hello, thank you for the quick reply.
I do have flashing led's on either side of the door but they're only app. 2 volts, flash every 3 or so seconds.

I understand a reed switch idea but I think that might be overkill to install since I'd have to take the door cover off, run a power source through the door hinge etc. I might create a source for more trouble down the road.
Ed

Have you thought about using a 555 timer triggered by the LEDs? It can basically convert the pulsed output of the LED to a constant output that would keep the unit powered up only a few seconds after the alarm is disarmed.

Hi,
I don't know about this component. Should I do a search on the internet?
Basically, if I understand this right, I could use this modul in connection with another power relay to use it as the switch for it?
As long as the alarm supplies power pulses to the LED's, it would continously send a signal to another relay that then feeds the shock sensor power?
Thank you again for the reply.

Hi there again,
I kept looking into the 555 and while I did that, I came up with another, easier idea. I'll hook the DEI 504 up, permanently and wire the trigger wire + to one of the factory alarm + cables. There aren't any empty ones so I'll use diodes to double up a connection. This way,when the alarm is not activated, the shock sensor keeps sending trigger signals to the factory alarm, but it won't do anything since it's not activated.
I will also install a toggle switch to delete the factory horn, which is also a input to the GPS unit. I'd cut the wire after the input wire hookup. This way, I'd have a silent alarm as well. I won't need any fance logic components.
I hope it made sence, the way I explained it.
Ed

We aren't installing it, it only needs to make sense to you.