heater fan

[hrsedrvr]

i have a 1996 gmc 2500 c/w a/c and i can't get the high speed on the fan to work

also it's a diesel so it dos'nt get real warm at the best of times

on real cold days the defroster can hardly keep up

i changed the switch on the dash and the resistor on the heater box but no luck

the local gm dealer said the only other thing could be a wire somewhere

it would be a nightmare i think to try and trace it out

anyone have any ideas?