For my Sei I used the middle bumper mount bolt, just above the middle of the waste air ducting.
Seems to be a pretty solid earth, although I've not run a current through it for any length of time - just wired my amp up to make sure that the cabling I laid at the weekend is sound.
Sei SX, 1108cc MPI, Calypso Orange(!)Mods so far: Colour coded (blue) gearstick gaiter; Abarth alloy tax disk holderProjects: ICE install (v. soon), exterior makeover (a little later)Wanted: 01-on Sporting Front Bumper, PM me!