Marco's advice is very good.
The cable from the negative post of the battery should be connected to the frame. Over time the moisture, dust and grime can work its way between the cable and frame and the current does not flow as smoothly as when the connection is clean. Scuff the mating surfaces (cable and frame) to remove the grime and then connect them back up. There is also a ground strap that connects to the lower bolt of the starter and runs to the frame - clean that up as well.
I've also experienced the grime on my fuse panel. Same cleaning principles apply, get rid of the grime and the current flows. When working with the fuse panel, disconnect the battery cable first thing and hook it back up after you finish cleaning.
Ricardo Maestas - Bio Page
1999 LX 470, 1971 FJ-40