My 2 year old GSD seems to be suscetible to getting foxes mange (scarcoptic).  We've had him on advocate for 3 months but saw no improvement, had one month's dose of Stronghold and have seen some improvement but some patches are still very sore.  I'd like to supplement the stronghold with something to see if we can get rid of the last few stubborn patches (it only seems to affect the edges / tips o0f his ears) and someone suggested Benzyl Benzoate.  I am not sure whether it needs to be diluted or how to apply it so would appreciate any advice.

It might be of interest to you to have a look at this site where this dog was treated successfully with natural remedies:
http://www.irishretrieverrescue.com/2008pros/monty2.html

Benzyl Benzoate  is well used by GSD owners for years
I would try it.  just shake bottle and apply