stereo stays on until door is opened

I see now that the The12volt.com example answers my original question, as the example uses a 1A diode.

Here's what I have worked up for a schematic using the single DPDT latching relay...

and with a delay circuit to keep the circuit from immediately latching before engine starter cranking...

Since my car's accessary wire is isolated from other accessories, this circuit should work..

And on my car with the delay circuit..

Maybe, but my car has a newer replacement ignition switch.  The old Porsche part number has been superseded by a Porsche one with a 944 prefix. Anyway, the ignition switch currently in my car does interupt the stereo accessory wire during starter cranking.  BTW, those old ceramic fuses were replaced by Porsche starting in 1985.

Here's a view of the prototype circuit I built.

Component list

RL1 – DPDT Latching Relay, 12VDC, two coil, 2A contact rating, eBay, about $7 with shipping

The time delay can be adjusted by selection of the capacitor and resistor values. The resistor’s value is also a function of the relay coil’s current draw and the transistors gain rating.