Air/Heat only blows on 4 (high)

I have a Isuzu Rodeo LS (I think) 1996 that has 181,000 miles on it with a V6 engine. I'm not really sure what I can do but my air/heat will only work if I turn it up to 4 (high). I want to change it out but its really hard finding a 1996 thats local that I can pull the part from. Am I able to change the part myself and if so what other vehicles could I take the part from? Do you know why it only blows on 4 also? Thanks