'99 saturn sl a/c only blows cold when idling high

we just replaced the compressor and recharged the a/c and now it only blows
cold air when i'm actually driving (particularly when i'm on the freeway).
when idling, it only blows hot air. we were told my many people that it's
a switch..but which switch? where do we start looking? i can't afford
the dealership or to pay a mechanic. my friend is very competent and just
needs a little more direction as to where to look.