FIY, this is how our GPS units are designed:

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On the "ignition" input the TVS is a big 1.5KE33 and none of the ~480 units failed me. The other "conventionnal" I/O are either "ground inputs" (the ones that fails often) or optoisolated. I had a few units who failed on those opto inputs but the TVS is a 0805 SMT. If I came across a failed one I'll do an autopsy to find out what failed...

HTH