Catching Referring URL in 404 page with from URL with .html extension

Question

Hey im trying to catch the referring url from a custom site which uses friendly URLS.

now if i do for example a invalid url

basepath/TEST/TEST

The url stays in browser but content switches to 404

but if I do

basepath/TEST/TEST/index.html the url changes to 404.aspx

so i cant get the referring url

any ideas ?