<VV> batteries

[MarPack57 at aol.com]

Working on a 63 vert. A month ago brand new 51R battery from Wal-Mart. Dead 
in two days. Last evening got a replacement (same battery code etc). Installed 
last night, today at 5PM dead dead dead. Only lights that are in-operable are 
the brake lights. Should I suspect the switch? Or????
 How does one track this down, Have already checked the glove box light, 
lighter, high beam switch, has new cables etc.
  Ed in fla