97 sport trac no heat

Hello Explorer forum. I just bought a 97 Explorer with the 6 cylinder engine and I have a few problems that need to be worked out.
To start, the heater blows but does not get hot. What all should I look at to remedy this? I figure I can replace the thermostat cheaply enough but if that does not work what else should I look at? The heater core is not too expensive. But how do i diagnose what is wrong and what else should I look at?
Next, the key does not unlock the doors. I am bringing it in to the dealer to get it fixed Monday. Should the ignition key open the doors? Will I be able to get a remote entry controller from a Ford dealer? It unlocks using the numerical keys by the handle.
Also there is some rattling coming from under the car that didn't start until I got it home. I figure it's just a loose muffler pipe so I am not too concerned with that.
Thanks for any help offered.
And one more thing, how do I get off the fake vents on the fender the previous owner stuck on there off? They look so awful on any car.

It may need a new thermostat. If that doesn't help take the 2 heater hoses going into the firewall loose and back flush the heater core.

Could you elaborate on that? I have never flushed out any kind of anything. And it's pretty cold out. You say there are two hoses that should run into the firewall but what do they look like or where do they come from? Do they come off the engine block or the radiator? The dealer I bought the car said they will do any work on it for the first year for 20-30 an hour so I am going to get them to do it most likely.