the blower fan on the suburban quit working recently. i had switched the hvac control to "off" for the first time in long time (trying to listen to a far away AM station). when i turned the heater back on the fan wouldn't blow. the slider hasn't really changed anything for the last two years, aside from making a vacuum hiss in anything except one position, i think "bi-level". now the fan won't turn on, i still get a little heat out of the vents, but it is not blowing. what do i do for trouble shooting this?