My horn goes off while driving on my 2007 Jetta. It would go off if I press the button on my key but then it comes back in while driving. What could be wrong?

suggest checking if 12 volts goes to horn wire at horn end when horn pressed from inside vehicle. A 12 Volt probe {google it} can be a cheap checking tool for this.
If 12v volts is switched when button is pressed {could be earth being switched if it is then place probe on constant 12 volts and negative lead on switched earth wire} then the horn itself is faulty.
If no voltage switched then horn relay needs to be checked. check if voltage at horn relay coil contact when button pressed.
if yes then relay may be faulty. if no then horn switch could be faulty.
you can check switched voltage also at horn relay switched contact when relay plugged in. To see if relay is doing what it is meant to do

there is a shorting problem in the horn button under the steering wheel.
the steering wheel will have to come off to fix it
find an accredited service centre that has experience in your make of vehicle

You can find the reset procedure here

https://resetoillight.com/volkswagen/oil-change-light-reset-volkswagen-jetta-2005-2006-2007/

With the key in the ignition (but not turned), press and hold the Odometer Reset Button (0.0) on the right hand side of the panel. While still holding that button, turn the key (although not far enough to start the engine) and then press and hold the Minute button on the left side of the instrument panel. Release both buttons at the same time. Try to reset the light as close to the actual service time as possible, because you don't want to risk any problems or confusion. I hope this helps.

You need to replace the door jamb switch. Little button on the door jamb that is pressed in when doors are shut.

replace the horn.thesecurity system should have a seperate horn to the normal "tooting" horn.

I just had this problem as well... thank god I had a handy AAA guy to my rescue, who used a flat head screwdriver in the ignition to get it to turn a bit, which caused the white lock I could see (that holds the key in while driving) to release... and then I could insert the key... Though I notice this is now becoming a reoccuring issue... so I think the ignition needs replacing...

First make sure that all the steering wheel assembly is fitted as it should be. otherwise u can stretch the springs a bit and this will increase the gap

Good luck

sounds like there is something in the wrong place in the horn button. it makes contact when you turn left just as if you had pushed the horn button. i wonder does your airbag light ever come on, i mean not just in the startup sequence, that's normal. probably you need to take the horn button part of the steering wheel apart and get things back in the right place or maybe replace some parts, but because of the airbag being right there too it's not something i would recommend that you do yourself. accidentally setting off an airbag is not a good thing.

you may need to put a relay in it as the current from the horn may be back feeding up the immobiliser horn circuit and causing a short. Sounds like the immobiiliser has its own source of power seperate from the factory circuit When you press the horn the power goes to the horn and up the immobiliser horn circuit and it blows the fuse.