How can I fix my 2005 Astra convertible's climate control?

The climate control is going haywire- doesn't stay on eco
when I set it. Goes to auto & temp display goes up & down erratically.
The fan insists on going to max & even when I turn it down, it goes back to high again.
There is a dreadful 'clicking' noise coming from the passenger side glove box area, so much so that my friends won't travel with me but opt to take their own cars instead.

I'm in Melbourne, Australia.