1) Probably just water condensing on your a/c condensor and melting off slowly. Higher humidity = more water. See if not using your a/c keep the water from building up.
2) If when you turn on your blower and you smell antifreeze then it's obviously not water and it's antifreeze. If this is the case then you've got a bad heatercore and it needs to be replaced.
3) The least likely cause could be from rain water leaking in from the vents in front of the windshield. But if you have water building up on dry days then this isn't the cause.
To get the water out find were the water is sloshing around. Some cars have a basket or box around the blower. You may just have to pull the basket out and drain it. It all depends on what the cause of the water is. If it is found to be water dripping off the condensor then there's probably something blocking the drain tube that goes to the underside of the van. It could be some dirt build up or even some trash that got sucked into the box and stuck over the drain. let me know if I was helpfull.