The first issue I had to resolve was that of the plugin being blocked by adblock. As someone else on the forum pointed out this can be solved by changing the name of the plugin files.
Now it is correctly affixing links with the affiliate ID however the geolocation doesn't seem to be working. This is the main thing I need it for since Amazon is a bit silly when it comes to linking to different versions of the site.
It is not affixing the affiliate ID to anything other than the .co.uk for me and all other links (.com, .fr, etc) are simply neither being changed to my local .co.uk nor being affixed with the affiliate ID.
Looks like a very useful plugin but clearly something is not working correctly.
Also, I am not particularly clear why it adds 'exec/obidos/ASIN' into the link as I have not seen anything else use this. Usually it is just /dp/ followed by the ASIN. I modified your script to use this and it still appears to function. To my mind 'exec/obidos' looks a bit dodgy. Certainly it would put me off the sale without stripping the link first. Maybe I am just paranoid.
Lastly I have just noticed that the syntax you use is /affiliate-20 when virtually every affiliate link I have seen uses /?tag=affiliate-20. Some sources online (granted old sources) indicate that the former does not work and that it requires the ?tag= first.