Where is the cabin fresh air filter on 1991 gmc 2500 vandura

SOURCE: GMC Vandura 1983 6.2 litre diesel automatic transmission oil GMC Vandura 1983 6.2 litre diesel automatic

Your dip stick tube is your fill tube for your transmission . Make sure your transmission oil is at opperating temp before servicing some instructions are written on the dip stick , Use a small funnel , Add a little at a time (cup). Use the proper fluid , check regularly

SOURCE: cant find cabin air filter on my 99 gmc yukon sle

It would be located behind the glove box, if your yukon came equipped with one.

SOURCE: serpentine belt diagrams 1990 gmc vandura

My sincere apologies for the delayed response.
I am personally going back to answer your unanswered questions.

I do not know if you still need this information, but I am going ahead and answering it anyway. You may want to Print the Diagrams for future use.

CLICK on the following LINK. It has the Belt Diagrams you will need.
If you would: Before Clicking: Let me know if this helped you, or if you have any additional information or questions. Feel Free to contact me at FixYa.com!
Again my apologies...
http://www.2carpros.com/car_repair_information/year/1990/make/gmc/model/vandura/1990_gmc_vandura_drive_belt_routing_diagram.htm

SOURCE: where is the air cabin filter in a 2004 gmc sierra

I can only advise you as to the replacement of a 2002 Sierra, under the dash behind the glove box compartment is a plastic panel held on with four screws, remove this panel and the cabin air filter should be easily identified and replaceable, if you purchase a filter from an Auto Zone Auto parts store or a similiar parts store, they will advise you of whether your vehicle is equipped with the filter/filters and also where its location will be.
Some vehicles are equipped with (two) cabin filters, and also I am told that if you purchase an OEM part(Original Equipment Part) instructions are to be included in the box to further assist you.

SOURCE: Where and how do you change the cabin air filter

I'm not sure but i think 03 and later yukons dont have them, I have an 05 yukon and pulled the cover off only to find solid plasic and nothing else to remove, certainly no clips to undue to remove anything. I looked and looked for about an hour. After some research on the net, it looks like there isn't one. strange they sell the part for my year. Either that or im looking in the wrong place.