1) I had a similar idea when I bought my 08 Accord V6 6speed. Instead of a gear indicator I wanted to add an aftermarket shift light. I discovered, after I had already purchased the light, that it was not possible to tap into the required wiring since Honda had begun integrating all of the circuitry in the dash. I haven't dug into the idea since to see if the new Civic is able to accommodate this idea (the gear indicator would be the same concept).
2) Why would you need a gear indicator for a manual transmission car? It's not hard to reach down and figure out what gear you're in by the current speed/RPM and the shifter position.
I agree it's not necessary at least for me. Our cars have enough screens to look at and you should be able to know what gear you are in without looking. But if it's what you want, who am I to disagree? If you do install it, be sure to post pics.
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