We have an 2006 sterling go to truck pro and they have them and they are cheaper than any one else we found and usually have a great deal in stock. Here is the part #PA5337. It should work since all sterling cabs are like m2 cabs . Hope this helps
The 08 model does have one one the passenger side. Take black cover off and it is right there. On the 06's,(8000 &9000) there are two. On one is the inside (large one) and one by the blower that is removed from the outside.