2003 Dodge Neon Defrost Knob

Hi! When I attempt to turn my defrost button to the defrost function, it jumps right back to the left. I have attempted to hold it in place to see if that will engage the defrost function, but that doesn't work. Please help. Winter is almost here and I need the defrost. I am a poor military retiree who is desperate; but do not have the funds to seek a mechanic's help!