The negative cable (or cables) connect to the frame. The positive one (or ones) connect to various combinations of fuses, circuit breakers, converters, inverters, isolators, disconnect switches, etc. Wire color is less standardized than one would really like (or rather, there are a few varying standards), and particularly for older used units could have been replaced or changed around from whatever it was originally. The important consideration is where the other end connects, not whether the wire is red or black or yellow with purple polka dots and glitter specks.
(For what it's worth, I happen to have a red positive lead, a black negative lead, and a comparatively thin white negative lead connected in parallel with the black negative one. I suspect the white was added by the previous owner for some reason, perhaps because the black one is broken, and I intend to make it go away the next time I do any electrical work/cleaning.)