Horn stopped working, dash light goes off when the headlight is on

Yer not gonna believe this but first things first get a new 12v battery, an optima to be exact.

When the 12 volt's low nothing but weirdness goes on, as to the steering wheel buttons, probably toast.

 

You may have a issue with the clockspring. You may also have a issue with the 12 volt battery.

Also, post's for the Prius V need to be in the correct forum here on Prius Chat.

I would check the voltage of the 12 volt and then go from there.

Best of luck to you and welcome to Prius Chat.

 

As dorunron states, you may be having an issue with the clockspring, otherwise known as the spiral cable. The spiral cable includes circuits for all the steering wheel controls including the cruise control switches, horn, and driver's airbag. As it fails, more circuits are becoming disabled.
As for the dash lights, the instrument panel dimmer wheel may have been rotated inadvertently.
Check the dimmer control on the dash, to the left of the steering column. If it's rotated all the way down, the instrument lights will turn off when the parking lights or headlights are turned on.

 

I believe in the US, the spiral cable is covered under warranty for 5 years or 60,000 miles, whichever occurs first. I'd verify this with your dealer.