Shut down voltage

[wahid]

I have a problem that has been nagging me for a long time and I hope one of you may have a solution. I installed an Autronic system on my C2 Turbo about 4 years ago, the system works great but when I turn off the ignition switch the car runs on for a few seconds before dying. The cause is that the voltage from the ignition switch drops from 13 volts to about 5 volts when the engine is runnig and the switch is turn off instead of going to 0 volts. If the engine is not running the voltage goes to 0 as soon as the switch is turned off. I can't find any circuits on the car that go from 12 to 0 volts as soon as the switch is turned off and I have tried every wire. Does anyone have a suggestion.
BTW I have the same issue with my 88 Carrera.