[thstone]

Check the current pull on the battery (called parasitic draw) when the engine is off and the everything else is turned off.

Its an easy test to perform: Place an electronic multimeter in series with the battery negative terminal and monitor the current flow. Do this by disconnecting neg battery cable. Then place the positive probe of the meter on the neg battery terminal and the negative probe of the meter on the unattached neg battery cable.

Car should be off with key removed and doors closed and so on - just like it would be if it were parked and locked.

The value measured will be in amps or milliamps. It should be less than 80 milliamps or 0.08 amps.

If the value is greater than that, then you have a short circuit somewhere that is causing the battery to drain quickly. To find the source of the power drain, leave the multimeter in place and start disconnecting each fuse one by one. When the power draw drops to less than 80 milliamps, then you've found the circuit that is causing the battery drain. Look up what components are on that particular fuse circuit and then perform further testing on each component in that circuit (disconnect one by one) to the find the source of the high current draw.