Visibility of a creative on the Back window sometimes is a problem due to the dust & smoke collection can you suggest a solution for this?

This problem is only evident in the MERCEDES bus model and this is because the exhaust in at the back on the bus. This is an in-built error in the buses and hence cannot be rectified. Although the buses are cleaned daily at the bus depot, we provide extra cleaning teams to ensure that the adverts are clean most of the time, if not all the time.