I've got a couple of Nextbase DVD players that I had in my old XC90. The main problem was fitting them to the head restraints because the Volvo ones don't come off and don't have the normal "poles" that you find in other cars
I found some bespoke brackets made specifically for Volvos - it was discussed in these threads. See if you can see anything in there that helps.
http://www.volvoforums.org.uk/showthread.php?t=124079
http://www.volvoforums.org.uk/showthread.php?t=105054
I haven't yet tried to fit the brackets in my new model XC90..........