Actually, I am very familiar with vehicle systems, voltage ranges and the spikes that can occur. However, when studying how other manufacturers have created their product to be installed in such a vehicle using the same features, I don't see much protection being used. Probably because the cost of the board and the likelihood of the malfunction and repair is not cost prohibitive. It almost seems like the opto circuit would be best in isolating one circuit from the other and maybe a 12 volt zener across the LED terminals would prevent damage to the opto. Any thoughts?