my heater control won't switch from vent to defrost mode

 

What are you driving?

 

just blows in floor and defrost sometimes when i go down hill will switch to vents

 

Likely have a vacuum leak somewhere.

 

It defaults to defrost if you lose vacuum.

 

Well, quite eating so many beans!!! Oh wait, that's not the air you were talking about?

Seriously, what do you have? My '96 Ram had a problem like that due to a broken lever. You may also have a major vacuum leak. All this is just guessing though with out knowing the model and year of your truck.