No Heat!

[Matt1104]

About a month ago i lost my mid blower controls on my 02 F-250. All i have is low and high. I changed out the resistor/plug that didn't fix my problem what else could be causing it? Also the last week or so my heat goes from warm to cool when i switch from head to floor or change from high to low back to high it gets warm again.

Thanks guys for any input