if the blower comes on in defrost mode only it could be the vacuum actuator at the blower motor assy.to access you will need to remove the under dash panel cover on the passenger side.you will then see the blower assy and vacuum actuator.watch this as you press the auto low button on the dash.you should see the actuator move.if it does not the chances are it needs replacement.you can replace the rubber end pieces.search on this sight.i beleive there is a source for these parts