No crank issue with 1999 Chevy Venture

Make sure battery has a full charge, use a digital multimeter, everything off, the generic spec for a full charge is 12.6 volts. Leave meter leads touching battery posts, have helper turn key to crank, how far does the reading drop or does it drop at all? The generic info says it shouldn't drop below 9.6 volts, if it drops way down, that isn't going to work. You could try jumper cables or jump box. Your battery connections are clean and good, all ends, not just at the battery. Your computer and security system has nothing to do with starter motor. I'm just talking about starter motor cranking the engine, not engine running. Some of the national brand auto stores will test the starter motor for you, if you take it to them, so you don't replace a good starter motor. At the starter motor, two voltage circuits, one comes from battery, hot all the time. The other circuit goes hot with key in the crank position, and is wired through the transmission range switch then to starter motor. There is a fuse that supplies voltage to crank position of ignition switch and the ignition switch itself. The starter motor grounds through the engine block. Any testing at starter motor, make sure transmission is in park or neutral and parking brake is set. Vehicle can't move. The engine isn't seized, correct?