Ethel, safety critical design is far, far more than adding a chip in parallel. For a start, the chips are running at some lower supply voltage, typically 5V, and a failure of the power supply will be catastrophic. There are THOUSANDS of failure modes to consider, and properly mitigate, in any safe design. The analysis would involve about 1000 pages of A4 and take a competent engineer, independent of the designer, 3 months of full time work.Then that would have to be confirmed by independent testing, including fault injection, and it would mostly have to be done again if the software needed to be updated, The analysis would inevitably produce unsatisfactory results, and so you would end up paralleling it with the main light and dip switches, so what would be the point?
Far better to be doing things like making the fuel gauge accurate, properly controlling the interlocking of fog lights with the headlights, replacing the troublesome hazard and indicator flashers, detecting bulb failures, maybe even monitoring brake temperatures, which can be done without the major safety consequences, and would best involve an Arduino. Loss of indicators, for instance, is only MAJOR, not CATASTROPHIC in the terms used in safety analysis, because it doesn't cause you to crash in a dark night on a twisty road, so the integrity requirements are much less. And there is only one basic precaution needed to make a better fuel gauging system using the normal sender. That is, limiting the voltage and current fed into the sender, even in fault conditions, to below the intrinsic safety limits.
And how about another Arduino idea, an average speed meter? If you come across average speed cameras at road works on the motorway, you press a button as you pass the first camera, and keep an eye on the indication, such that you pass the next camera with it showing 50 (or whatever limit is set) even when you may have momentarily exceeded it in between, to make up for being slowed down sometimes? That could speed up progress and save your licence. I would suggest a GPS module for the speed, but you can't legally use it as the main speedo.
But you CAN use an Arduino, and a sensor on the gearbox instead of the cable drive, as part of your official speedo. The Arduinos usually have an area of EEROM where the recorded mileage can be saved periodically, as that must be maintained even when the power is off.
Then there is control of the heating system, electric mirrors, synchronised independent wipers, wiper delay, monitoring of anything that takes your fancy, as on modern cars. Tyre pressure monitoring if you can fit ABS speed sensors to each hub, to sense the different speed of a deflating tyre. All in all there is plenty that you can do safely and legally.
But one thing you CAN do with the headlights is to use solid state switches instead of traditional relays, as long as you observe strict duplication between left and right. That lets you use TWO fuses instead of FOUR, one feeding left and the other feeding right. Safety requires that the left and right side switching is in separate boxes. The protected switches look after local faults, so you can feed each side of the lights with its own fuse from the starter solenoid, keeping the unprotected wiring as short as possible. But there is no software or complexity involved, the dip switch simply feeds control voltage to the solid state switches, which always have battery power. You would split the dip switch output 2 ways at the switch with resistors in the lines such that a short on one side did not affect the control signal to the other. The light switch and dip switch would only be handling tens of microamps, they might need a ballast resistor to load them up to at least 10 milliamps to keep the contacts clean. That scheme actually improves safety, because it has reliable redundancy, unlike the standard system, except for the light switch and dip switch. No software, no logic gates etc, only some resistors and zeners to protect the control inputs on the switches. The proper parts will be rated to withstand alternator load dump.
Is that enough SAFE ideas to be getting on with?
