I just replaced the belt on my rear A/C and I wanted to share the info on it. It was, in fact, very easy as I will detail below.
I am running a 2006 140" passenger with rear air that I bought used. I know that the van spent a good portion of its life in Texas and other parts south. Looking through the service records I got with the van, I saw that the belt was replaced at least twice before and in fact the idler pulley (tensioner) was also replaced at least once.
So, initially got the van and ran the rear A/C and it was squealing, but that went away. Then my wife says, the rear A/C is not working, please get it fixed (had already had to charge / recharge the front system). I went to look under the van at the belt and - belt is gone missing. So I looked here at this thread and it indicated the Dayco 17380. I did some calling around and nobody had it but they could get it and it was going to be something like $16. Then I went online. Found the 17380 belt at Rock Auto for $5.40 on closeout - great. So I put one in my cart and then I thought, well, I will just look through the parts recommendation on Rock Auto for this part to make sure it matches. Recommended A/C accessory belt they list is a Goodyear 17391. Hmmmmmm... Well, they have two of those on special too for about $5.40, so lets get them also. Two days later all three belts show up. Total with shipping was $23.xx.
Out to the van. Hey, that bolt head on the tensioner is not a torx (E-star). OK, it is just a regular 13mm head, no problem. Loosened the tensioner up, slip on the 17391 belt, apply tension with the adjustable cresent wrench as noted and tighten it back up. All done. My 7 year old helper with limited English skills says "All done?" and I say yep. He is like - wow, that was easy. We take the van out for a test drive and everything works and no noise whatsoever. Now I have a couple of spare belts for the inevitable replacement job.
So, the takeaway here is that you need a 17 series belt, but a 380 or a 391 length will do fine. At least for the 391 length, getting the belt on is very easy and it tensions up just fine.
Chris