There is a product called Back2Natural that people swear by.
Its meant to restore colour and is easy to use regularly.
Its especially designed for replacement hair.
TopLace will be stocking it soon.
In the meantime, a Google search will show you where you can get it from.

I know you say its not the sun fading your hair, but that you do live in a sunny climate.
I have light brown/blonde hair and have recently returned from NZ.
I was there 5 weeks and had to darken/colour my hair twice because of exposure to the sun.
I only have to do that occasionally back here in the UK.
So I think sun can play a factor in fading and a good sunscreen for hair is recommended.
Once the hair has become 'bleached' by the sun, it becomes more porous and likely to absorb the colourant in the C-22.

If you believe that the C-22 is causing the problem, you should try using pure alcohol instead.
Its colourless, but won't smell as nice!
Its readily available in 91% and cheap compared to specialist products.
If you can get 99% alcohol, it works even better.