I do not believe any year came equipped with a drain plug. Your choices are to **** it out, or drill and tap the case to facilitate routine draining (I don't believe removing the carrier assembly is a realistic option). I chose the latter and had success drilling and tapping the case. As a result, I now have the ability to readily change the lubricant which many profess to be a critical maintenance issue on a Viper.
I acquired a flush fitting 1/8" plug from McMaster Carr;
www.mcmaster.com or telephone them at 404.346.7000. The part number of the pipe plug is 4534K11.
good luck ......