You can ground it anywhere... a ground is a ground. the positive needs to have constant power, im not sure but the main fuse might get a cut of power when trying to start the car.. like an accy. wire, which would cause it to not function while trying to start it..

I got mine when i purchased my 4.88 rear end, i had the guy throw it in to make a deal... he never replied when i said it died on me.. :/