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Which direction should the diode, cathode or anode, be facing for a car security alarm wire? Below is a diagram.

When the doors are closed, the wires are testing at 10.1 Volts, settings on the system could be set for either negative or positive door trigger input.

A system that tests wires as always on when doors are closed means it's a negative door trigger?

enter image description here

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That diagram is not very useful, since all the switches depicted are in parallel, so you only need one connection, but if there were several lamps to be lit separately e.g. in the trunk or under the hood, you'd need the diodes to prevent a single switch from pulling all the circuits on (low) at the same time.

For that situation, the diodes as illustrated would be correct. The cathode, as Eugene points out, is the end with the stripe, and this is a negative side switch - the connection you are monitoring is held high by the bulb until a switch closes.

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    \$\begingroup\$ the diodes also prevent 12V flow flowing into the input. that may be a requirement for the alarm module. \$\endgroup\$
    – Jasen
    Jun 10, 2019 at 21:38

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