I'd like to start / stop a piece of software running on an in-car computer depending on the 12V_IGN rail going on resp. off. This happens when the ignition switch goes in / resp. out of position RUN. See these two scans from the original service manual. The computer corresponds to the radio on the schematics:
During my dry tests, I simply used a switch (connecting the IGN pin with GND), a pull-up resistor, and a CD4093B Schmitt trigger:
The output of the trigger goes to a level shifter because the computer expects +3.3 V on its GPIO inputs.
Now I'd like to know whether I will still need a pull-up / pull-down resistor when using the 12V_IGN rail of my car as input, or I can attach the IGN input pin directly to the rail.
UPDATE: In the service manual, the power junction block stands for the permanent (aka standby) +12V rail, and the wire labeled BRN in scan #2 is 12V_IGN, as it is only hot when the ignition switch is in position RUN.