Unless you happen to know for fact that all these things failed at the same time, you have to separate them as separate events and separate causes. The horn problem - Remove the air bag assembly from the steering wheel and disconnect the wire for the horn off the bottom. If the horn stops when disconnected, then the contact in the air bag assembly is stuck closed. The power locks diagnostics gets a bit detailed but they are unrelated to the horn. The truck circuit - check for 12 volts to the truck release solenoid.

I could make two paragraphs here about checking the battery drain, but instead, let me direct you with the correct terminology so you can google up many many pages on how to check for a "parasitic drain". Use the search term "battery drain or parasitic draw or parasitic drain and you can read in detail hhow to narrow the cause down.

The check engine light is located on the dash and stays on if the computerized engine control system detects an engine performance or emissions problem.

Check the vehicle for fault codes to see if there are any sensors were involved to the malfunction of the engine.

1.Local auto parts stores such as Autozone ( not in California) or Advance Auto parts will scan and retrieve any diagnostic trouble codes ( dtc ) stored in the powertrain control module for free of charge..

2. Amazon.com has the code reader for less than $21.00 dollars.

3.Re-post the dtc's in here " answer question " for further assistance.

Note: we need the dtc's to identify the sensor, there are a dozen different sensors to operate the car.

Visit this site and see all the different sensors.

http://www.freeautomechanic.com/car-engine-sensors.html

Navigate to this site and see the code reader
http://www.amazon.com/.../...imh_gw_p263_d0_i1

Rough running, misfire fault codes stored in engine control module.

Please detail what the LED's do, any response from any button? Does the air blow out at all and where does the air blow from? Do the LED's blink? Or, does the entire climate control display remain dark? When was the last time it worked and did all things work normally, ac and heater?

Obtain a loan free radiator pressure tester from the local auto parts stores such as Autozone, O'Reilly or PepBoys. Follow their instructions and pump it an adequate air pressure 12 -15 psi or the allowances on the radiator cap. Observe the leaks and perform the repair.
Note: do not OVER-PRESSURE than the pressure allowed in the radiator cap.

Do a pressure test while the engine is cold. Repair the leaks, it will bring back the heat due to insufficient
of coolant.

Engine Oil and Filter
Check level with engine off at every fill up. For maximum engine life, change oil and filter every three months or 3,000 miles or as directed in your owner's manual. Use specified oil grade and weight.

I am assuming your vehicle has a keyless entry and starting system. What this refers to is your security fob battery. You can get it replaced at most auto parts stores. The vehicle is telling you it is low so you can replace it before you are locked out, or not able to start your car.

Replace the Headlamp Driver Module (HDM) Relay in the underhood fuse / relay panel.

headlight relay replace no

i don't have a pic. but i can tell you approximately where it is at...depending on the engine it will be either on the passenger side of the engine compartment attached to the timing gear cover it isn't that costly of a repair. and the other engine model it is located on the driver side engine compartment on the engine on one cylinder head or the other usually as you are looking down on top of the engine from directly in front of the vehicle the passenger side you can see the wires and then the sensor.its a flat button type.a good flash lite will aid in seeing it.the sensor in your vehicle trips the engine lite but doesn't cause any more damage than that,but that's annoying enough. the sensor itself is only around 20.00 dollars and takes a garage about 1-1.5 hours to change.sorry for no pics..

Excessive questions. See previous post.

Check the fuel level sending unit, it stuck and caused an erratic reading.

You should not have to add any coolant - if adding to refill weekly, then this indicates there may be a leak; either internal or external - someplace in the engine or cooling system.
Have an inexpensive cooling system pressure test performed - with the engine cold, or borrow a loaner tool (cooling system pressure tester) from any local auto parts store - to determine if there is any leak for repair.