I don't have any direct experience with the item but IIRC, at least one person on the forum has installed the brake light from this company.
http://www.jandlenterprise.com It is thin and probably weighs very little. It mounts with suction cups or adhesive but if you wanted to remove it for shows, I would think a disconnect plug in the cable located near the light would make that possible. I am not that fond of suction cups long for term holding power but maybe fabricating a bracket so some small but strong magnets could hold the light to the garnish molding would make removal easy.
http://www.rare-earth-magnets.com Another option might be to get one of the license plate frame brake lights. Those come if quite a few styles but I don't know if any of them are available for 6v.
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