burning smell from vents??

hi everybody, wife smelled burning smell from vents. she drives 94' Jimmy 4.3, 2wd. that was a couple days ago and failed to tell me, now there is barely any air/heat coming form vents. first checked the fuse block and fuse good. then pulled the speed selector switch and found the 4th speed wire slightly burnt(brown or purple), also pulled the fan motor, with power to it i saw/heard very small changes in the 1,2,3rd speeds. bought new speed selector and just ran the first three speeds and still not enough air flow to heat/defrost vehicle. also checked to see if there was clog, found some leaves/straw in near the heat core, used vacuum to clean out. after that there was a slight increase in the air flow. switched out fan motor with spare for 89' astro, but did not switch out squirrel cage , so don't know if motor is problem. need help from the experts, this forum help me replace a fuel pump for my jimmy, now i need help with this problem. thanks guys!!
Gary

Sounds like the fan motor was/is drawing too much electricity, probably because its just worn out. This probably also damaged/overheated your resistor pack. Pull the resistor pack and check them, check all your connections and go from there.