I just took my battery out and put small screening in the vent holes.  Alos, While I had my grill out and the instrument cluster out, I realized I could see right outside!  On my van  (86 with no AC), there were two approx 1 1/4" inch holes right in the metal parallel to the ground, but above the the drivers side headlights - got that. Pull your grill and look upward above the drivers headlights to see these two holes.  I drove two tapered rubber plugs into these holes from the inside of the dash.  next I need to block the open part of the freash air intake.
I had to pull my dash vents out of the van to clean them, they both smelled bad, - seems like the mice liked to hang out up there.
John with mousetraps in the van.