Having worn starter contacts is by far the most common cause of intermittent starter activation.
TOYOTA & SUBARU (DENSO) STARTER SOLENOID CONTACTS & PLUNGER REPAIR/REBUILD KIT
Check out: http://www.ebay.com/itm/TOYOTA-SUBARU-DENSO-STARTER-SOLENOID-CONTACTS-PLUNGER-REPAIR-REBUILD-KIT-/131152710170?fits=Make%3AToyota%7CModel%3APrevia&hash=item1e894f8a1a&vxp=mtr
Toyota Starter Part Number
28100-00100-84, 28100-07010, 28100-0C010, 28100-0D010, 28100-0D020, 28100-0W020, 28100-0W040, 28100-0W050, 28100-46090, 28100-46100, 28100-46140, 28100-46220, 28100-50040, 28100-62050, 28100-75090, 28100-75110, 28100-75130, 28100-75140, 28100-76070, 28100-76090, 28100-76091
This instruction is for a Toyota van but you get the picture. Thanks timsrv for doing this instruction!
