Funnilly enough I have been thinking about the same thing lately. I had a transmission fluid line leak and now it smells like burnt fluid when I turn on the AC
Here is a thread I found at Diesel Place Forum that is 6 pages long but talks about doing a retrofit.
I have not had the time to read it all the way through but at 1st glance it may have some good info on this subject. Let us know if it helped
http://www.dieselplace.com/forum/showthread.php?t=48973&highlight=retrofit+cabin+fi lter