Before tracing the wire, I'd suggest checking the two ends you know about (the generator and the battery) and make sure that the terminals are good and tight and the wire is not corroded/broken at those points. There probably is some sort of a fuse or fusible link at the battery end, too, that would be worth checking. As often as not, in my experience, it's the connectors that give trouble.
If the fuse is blown, it would of course be wise to check the wire with a meter to make sure it isn't grounded somewhere before just putting in a new fuse.
And, to answer your question, I don't know where the wire runs, but I would tend to guess underneath by the shortest reasonable path to the battery, both to minimize voltage drop and to minimize the usage and cost of the (comparatively expensive) heavy wire.
