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It sounds like the brake lights were stuck on due to a bad brake light switch.
When the alarm was going off was the head lights flashing?
Why want the car jump start
The head lights flashing is the factory alarm
which could have been active since battery level voltage got low
To turn it off you just put the key in either of the front door lock cylinders and lock and unlock the door
I would have the brake switch checked
if bad it can keep the brake lights on like you are describing, and can also keep them from working.
This part is not expensive, it's around 20 to 30 bucks
Will the on brake light kill the battery?
Sure it's a load.
If you leave the lights on it kills the battery, if the brake lights stay on it does the same thing and can drain the battery as well.
Any reason why I could not jump start the car?
I would try again, and turn off the alarm by using the key in the door like I suggested and see if checking the connection will help.
Also if the battery is that low in voltage, you may want to leave the car jump starting it run for 5 to 10 mins before you try to start the truck