1992 V20 - Looking to add a second second battery

[Petrus]

My 1992 V20 has a single battery located under the baitwell in the starboard side of the transom- it looks like there is plenty of room to add a second battery- The 1992 v20s had second batteries as options- anyone know where the battery switch should be located? The transom looks like the obvious place but would like to know before I start drilling. Thanks