"I've taken her to couple of mechanics who didn't have a clue."
OK, in Australia 1997 B2600i is like USA 1993 B2600i. So I can help, in fact - I know EXACTLY what the issue is. You have a bad blower resistor; your symptoms are classic. Sad that "a couple of mechanics" were clueless.
There's a real good write-up here - in fact I wrote it - see
http://www.mazdatrucking.com/forum/showthread.php/12985-Blower-motor-and-blower-resistor-check-out
Post was last edited on Oct 25, 2016 08:37. Edited 1
time.