New battery, no crank, no lights, no horn?

I have a 2001 Ford Focus, I've recently replaced the ignition switch, thermostat housing (over heating issues), and yesterday replaced the old battery. My car still won't start, nothing happens when the key is turned except a few lights in the dash try to come on, the radio will come on unless I try to use the air, whipers, or head lights, then it goes off like it can't do more than one thing at a time. The A/c, whipers, and head lights won't turn on at all. The door dings but that's about it. The positive battery terminal is somewhat corroded, could that be an issue? What else could be causing it to not crank?

Something got shorted, your terminals are corroded or you're getting poor connection with one or both of your terminals.

Why take the time to ask when you could have taken the time to clean the terminals. You suspect the terminal so you think it might be the problem, clean it and then do the other one.

Some Fords have a "megafuse" between the battery and starter. It could be blown. The other answers are correct too. It could be a no cost fix like corroded connections and quite frankly I bet that is the likely culprit. Spend $4 at Pep Boys and get a battery terminal cleaner. It is bullet shaped and has two (2!) brushes in it that you can use to clean battery terminals and all kinds of other electrical connections, including the ground - usually the engine block and or frame.