My father in law has a 97 ram 5.2 L and when he steps on the gas the AC switches from vents to defrost. I have read in this forum that it is due to a vacuum leak. Any idea which hose or at least where we could start to look for the bad line?

 

There are vac lines running out from the firewall on the pass side. The one you are looking for has an inline diaphram valve. That usually is the cause. If not it is the cruise diphram/line that runs over from underneath the battery.

 

I could be wrong, but I have heard that the problem can lie with a vacuum check valve that goes bad. It's where the vacuum line comes off of the intake manifold on the left side of the engine (facing the engine).