The rules of thumb for the Icelandic highland at wintertime is simple. It is closed.
After said that we can also say that driving in the Icelandic highland is very popular at wintertime, both off and on glaciers. To be able to do that you need a super jeep and a very experienced and skilled driver used to the conditions in Iceland in winter.
Organized tours usually result in better experience as well as a safe and enjoyable trip.