Hello, I'm Jason.I will do my best to answer your question quickly and accurately using the information you have provided.
Ok, before we go into a long process of diagnostics here, we definitely have to start with the basics. First: are you absolutely sure that the battery is good. Meaning a full 12.5v or better, and 500cca or better available?
I also need to know if your car is an automatic or stick.
Then we definitely need to make sure about the battery. Also, it sounds like you have a key-off battery draw, since these cars typically won't drain a battery down to a non-start condition unless about 3 weeks have passed. You ask about jump starting. If the battery in the car that is being jump started is too low, then jump starting won't necessarily work. Jumper cables can only carry a "certain" amount of amperage.
But again, it all starts with a good, hot battery. Most parts stores will do a battery test for no charge. If the battery tests well, and you're considering doing your own work, then you will need to have some basic hand tools and a multimeter available.
Key-off battery draw: when you shut the car off, and lock it, there are a few electrical consumers that stay energized. Some for only a minute or two, and some longer. Typically, only the alarm system stays on for an extended period of time. If anything else does, this will draw down on your battery. If you have a multimeter with a mA reading, then it's something you can do yourself with the following procedure:
Open your bonnet (engine hood), and then manually re-latch the latch using a screwdriver or similar (we're re-latching so that the alarm computer will think all latches are closed). Open driver's door, and also re-latch. Disconnect negative lead from the battery. Set your multimeter to the A or mA setting, and place your meter leads between the battery cable and battery post. You should then see how much current is drawing on the battery with everything off. Write down, or take a note of the amount of current (ex: 550mA). Then open your fuse panel cover on the left (driver's) end of the dashboard. Start with fuse 1, remove the fuse and look at the meter. If there is no change, reinstall, and move onto fuse 2. Repeat this process until you see a significant amp drop on the meter. You will then know the circuit that is drawing down the battery. Look at the fuse legend, find out what is on that circuit, reinstall the fuse, and then unplug any electrical consumers on that circuit until you find out what specific component is drawing on the battery by unplugging them one by one until the amp reading on your meter drops below 50mA
Regarding the horn question, this circuit is only live when the key is in the on position.
Ok, then our next step is as I outlined in a previous post:
Without actually being able to use a test light to see if the starter is receiving a start signal from the ignition switch, I can only give an answer based on items already tested. From what you're describing, yes, I would put a 90% chance on a new battery starting the car.
So, now we are to the point where you need to use a multimeter or test light to determine if there is a start signal getting to the small wire on the starter. If there is a 12v signal to the starter, and both electrical connections to the starter are clean and free of corrosion, then replace the starter. If there is not 12v getting to the starter when the key is turned, we will have to do further testing, which I can outline after the signal test is performed.
The starter is towards the front of the engine bay, just behind the radiator. It's bolted directly to the transmission bellhousing. Follow the main positive battery cable downward, and you will find it.
I highlighted the main power terminal in red and the starter signal terminal in blue
The starter, and any testing that I speak of can all be done with the car on the ground. The starter is located about halfway down the front side of the transmission bellhousing, in the area I indicated here in red
You will need very few tools. Possibly a 10mm socket to get the steering reservoir (green cap) out of the way. You will also need either a test light or multimeter to probe the small wire connection that I spoke of previously. At the same time, you will need another person inside the car to turn the key to the start position while you see if the test light illuminates, or multimeter shows battery voltage, when the key is turned.
Let's just start with testing before going through the removal and installation process.
This needs to be done with the battery in place, so that you can test for voltage. If you're using a test light, you connect the alligator clip to a good ground, even the negative post of the battery. Then, if you're not comfortable back-probing the starter signal terminal to the starter, just unclip the signal wire from the starter, put the tip of the test light into the harness end, and have your assistant turn the key to the crank position. Tell me if it lights up.
Then we will proceed to the next step, which may or may not be to remove the starter.