If it not a wide split just a long one. two different ways to deal.
1. Seal off the split and cover with plastic vent.
OR
2. White trim strip as used around pvc windows all the way from front to back shape ends to look as though its purpose made. Coat back of strip with mastic [ two persons job] one holds strip to wall the other screws it to the wall ensure its level, use screw caps to cover the screw heads. I did my last c/van and it looked quite smart when done. Ensure you seal off the split before applying the strip.
Bob
November 2009