heater/defroster/AC not working WHY ??

ok everyone, i have a 89 cherokee pioneer with A/C
the heat defroster and ac have been working fine till yesterady
i go to turn it on and nothing happens i can here a noise every now and then from the passenger under-dash, but no ac or heat comes out
when im driving, a FAINT wind comes out of the vents when its on, but at red lights, its dead again. does anyone know what the problem is ? when im driving and it barely blows, the heat is hot and the AC is cold. so im confused ? im not good with A/C and heat but if i knew the problem i might be able to fix it. thanks. matt

When you turn the fan selector to 'High' does it blow? If so, you've got a dead resistor pack in there. Search for that term here to learn how to fix it...

but the thing is. when its on low and im driving. it will SLIGHTLY blow air. but when i turn it to HIGH. it crapps out ? what should i do ?