Truck won't start, even though the battery seems to be good. on 1994 Mazda B2300

One possibility is a loose connection somewhere, but I haven't found anything yet. Lights sometimes go on, but other times, not. If the lights are on, it acts like a battery drain from a weak battery when I turn the ignition or some other electrical source. However, battery tests as good, and doesn't seem to show any drain when turning the ignition according to my multimeter.

Asked by for the 1994 Mazda B2300