My heat pump only works in cool weather, not at all in cold. It only blows cold air in temps below about 45-50 degrees.
I don't think the aux heater and/ or the emergency heat are coming on.
Where do I need to go in the unit to check these components?

Thanks

A friend of mine had a problem very similar to what you are talking about. He blew a fuse in the lower pump system and foolishly didn't call a repair person and ended up running his emergency heat all winter and had a very cold house!