Just seal it, because the crazing is in backlit areas it is very difficult to patch it and maintain the same level of opacity.
As standard white bulbs will shine through the crazing, you could use some coloured leds in those areas to disguise it.
Without causing any further damage gently clean it if you can before sealing it.
+1
A GENTLE clean with a soft moist rag with windex sprayed on the rag - NOT the backglass. This will arrest any further issues and you can then decide if you want to touch it up.
