You surely have a drain on the battery, that is why it always goes flat. In other words, a short in the wiring system somewhere. You need to do some basic electrical troubleshooting to find the drain (short). It is a process of elimination. If you are at a loss how to go about it, any friend experienced with simple electrical work can show you how.
That may not be the only electrical glitch you have, but fix it first. If you don't, it will make you miserable and cost you a lot of money in batteries, because draining a battery ruins it.
