Make the rum sauce using 1/3 cup of rum, 1/3 cup of dark (or light) brown sugar, and 1/4 lb butter.
Pour the rum into a small sauce pan over low heat. Add the butter and let it melt while you stir it around. Add in the brown sugar and let it dissolve while you continue to stir. Once everything is melted and dissolved and warm, remove from the heat.
Do not let the sauce boil or you will end up making candy.
If you'd rather use a maple sauce instead of rum, use 1/3 cup of maple syrup, 2 TB of dark (or light) brown sugar and 1/4 lb of butter.
Pour the maple syrup into a small sauce pan over low heat. Add the butter and let it melt while you stir it around. Add in the brown sugar and let it dissolve while you continue to stir. Once everything is melted and dissolved and warm, remove from the heat.
Note: Do not let the sauce boil or you will end up making candy.
With the ham wrapped in plastic (to contain the mess), fill the injector with the warm sauce and inject as much as you can at 1 inch intervals. Some of it will leak out but much of it stays in there and flavors the inside of the ham.
When the injecting is finished, remove the plastic.