So I know this is a pretty old thread- but I didn't see our discovery mentioned so thought I'd add it for anyone looking.
We just got a '02 classic we are currently "desmelling".
One unlikely place we discovered that was holding a lot of smell were the screens in all the windows, door and fantastic fans.
It kind of makes sense when you think about it- the screens are like coarse air filters- lots of airflow passing through them = things start sticking to them and they take on a smell + tons of dirt.
Since the screens are so thin and dark its hard to see if they are even dirty. However, when you remove them, you quickly can compare the edges that have been tucked into the frame vs the main surface and I'm sure you will see a dramatic color/filth level difference.
IMO its not worth cleaning them, rolls of screen are so cheap from Home Depot, just take them out (but save them to use as templates), buy some new screen, cut and reinstall.
It didn't remove the entire smell but it definitely appeared to be a major source of it. Secondary to that was the curtains which we sent to the dry cleaners. The blinds may be guilty as well, but the PO had them closed and tied up almost all the time, so they are not as bad.
As a note I don't think our smell is so much "horse barn" as much as just smells from the PO. (But after reading above I'm definitely going to sniff around the vinyl too.)