xpath document() function in biztalk map

I have written an XSL stylesheet to transfrom a message which requires data from external xml file. I use the xpath document() function to load the external file, and when I run the transform with the VS.NET debugger it works perfectly.

I created a biztalk map and set the Custom XSL property to use my
stylesheet. However, when I run the Test Map function with the same input document, the transform fails becuase it will not the load the external data.

I did a lot of testing with different external files in different locations
(web, file system etc. ) and it seems that when run in the context of the
mapper, the xpath document() function does not work at all.

Has anyone else seen this behavior, or is there some reason that the
function is not available?

Any help would be greatly appreciated.

Thanks,
Seth