Be careful not to have this turn into that recent Honda civic case here where all the obvious causes were not checked first. Don't jump to conclusions.
Don't ignore Tosh's advice above: "So you need to first determine if the high current-carrying wires and ground have low enough resistance and good connections. Try paralleling the wires with jumper cables to see if it makes a difference". It could be a dying starter AND a bad wire - the jumper test might not work.
A year ago I installed two rebuilt starters and thought I got duds. Turned out to be internal main wire corrosion that didn't allow enough current to pass to the starter. The starters died about one week after installed. The wire was the underlying cause, but the rebuilts worked for a week before the low amps killed them.
Don't ignore Tosh's advice above: "So you need to first determine if the high current-carrying wires and ground have low enough resistance and good connections. Try paralleling the wires with jumper cables to see if it makes a difference". It could be a dying starter AND a bad wire - the jumper test might not work.
A year ago I installed two rebuilt starters and thought I got duds. Turned out to be internal main wire corrosion that didn't allow enough current to pass to the starter. The starters died about one week after installed. The wire was the underlying cause, but the rebuilts worked for a week before the low amps killed them.