I have a couple of threads on here about sarcoids on my mare (this
is the main one), so you could look through those. To summarize though, I highly recommend that you review the University of Minnesota Aldara protocol
for aural plaques. The vets there are extremely responsive to questions- your vet could contact them, or you could reach out directly (I emailed the main researcher about treatment, and she emailed me back within a day). I will warn you those, this direction is really pricey, and you'll need a prescription from your vet.
For a different approach, I would recommend you take a look at this woman's blog
about her mule, Lily. She's had great success, and was also very responsive to my questions about her treatment approach.
Overall, you need your vet's advice, but hopefully these resources can get you started. Good luck, it's not fun to go through- for you or the horse!