I bought the cabin air filter from amazon http://www.amazon.com/gp/product/B00...00_i00_details
and I just replace it but because it's my first time to do it, I noticed that the side of filter are different. one side was very dark, and other side was little bit dark. and I installed to face very dark side to the bottom, is it right to do it? It should face to top? or it doesn't even matter?

 

It's been a while since I did mine, and I don't recall. But Mann (smart choice, BTW) always ships an instruction sheet, and there should also be directional arrows stamped into one of the long sides. That is the direction of flow. I forget which way this system flows (up, I think), but you can easily tell from the dirty side of your old filter. All you do is have the arrow point away from the dirty side, and you are good. So if the dirty side was the bottom, point the arrow up.

 

The arrows on the filter should point up for air flow. I have a filter sitting on my work desk.

 

there is no arrow, but it says Mann asotop (something like that) on the side...

 

I don't remember it making a difference but, when I did mine that box lid cut the dog **** of one of my fingers.

 

When in doubt, put the grey side down. That's how aquarium filters run - charcoal first, then particle filtration.