I’ve got the same cold-air-on-feet problem in my 1991 Previa that I just bought in Washington and drove home to Alaska. Heater works fine otherwise but no warm air to feet. AC doesn’t work but that’s not a problem in Alaska. Cold feet are! Cold air amount depends on drive speed, not fan setting, so there’s a leak to the outside somewhere and a damper not getting warm air to the floor vents. Any advice before I take the dash of my “new” van apart?