Generator or has it been changed to an alternator? If generator the regulator might be sticking, an alternator can also create a parasitic drain on the system.
The brake light switch is a great suggestion, do you happen to have a glove box light? If so that might be draining the battery.
As suggested disconnect one cable and use a test light between the cable and the battery post. If it lights you have enough of a draw to bring the battery down. Start removing fuses or disconnecting items one at a time til the light no longer comes on.
