In the rear of the van, by the floor , on the driver side there is a vent that goes to the bottom outside. You can crawl under the van to see it. Remove the flaps on the vent and you can allow fresh air to be pulled in. I did this. I lined the passage way with closed cell foam and mass loaded vinyl (sound proofing). Inside the van a put a 5.5 inch computer fan (very quiet) over the opening to draw fresh air in. I was concerned about Carbon monoxide coming in , but my CO detector registers none. I also put a metal mesh screen over the opening under the van to keep the bugs and critters out
