Mind you mine is more than likely wrong, but I found a white material attached to my tail light panel when I got my car. I say this is more than likely incorrect as mine was rear ended at least 2x before I got it. But you asked what people are using. So I used white caulking compound (non-silicone type) as that looked like it would be the closest replacement to what I found. And though my two license plate light holders are slightly different from each other, I used what I had. No idea what to replace them with.