I just installed a new Williams double side direct top vented wall heater. When I put the cover on the heater it trips the vent switch after 5 -7 minutes. If I leave the cover off it runs fine. Help!

Check your gas pressure. It might be to high allowing the unit to over fire causing overheating/spillage/vent trip. I am sure you already checked the vent to be sure it is clear of any obstruction?

Wall heaters have to be vented exactly as specified in the manufactures installation booklet or you can have problems with tripping on the vent safety switch.
In the installation guide it will tell you to use type "B" vent , this is double wall pipe , if you use a single wall pipe it will not vent properly or if the vent does not terminate above the roof at the proper highth above the floor, it will not vent properly.
Wall furnaces are the most dangerous furnaces that are installed in peoples homes because the carbon monoxide gases can enter the living space if not vented properly and kill you. These gases are odorless and can cause vomiting, brain damage and death. Don't take a chance with this furnace , be sure it is in stalled properly. Good luck, Mike