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An inexpensive yet reasonably effective fix is to apply a plastic cleaner/rejuvenator type of product to the lenses. I normally use a Meguiars product for exterior plastics but on interior surfaces I use a cheaper, more watery product (CRC Protectant - perhaps not sold in your country). This what I apply to tail-lights.

Once I have finished a few interior surfaces with the CRC I then rub the applicator over the lenses. It seems to nourish the clouded lenses and return some colour to them. I tend to rub it in until the lights do not look or feel greasy.

Its not a permanent fix but if you do it every time you're applying the protectant somewhere, the tail-lights will generally be kept looking OK.