Power Drain

Hello, I recently purchased a 91 Pontiac GP, with 198K miles. It had been taken well care of and runs good. The problem is there is something that is drawing the juice out of the battery, to the point that there is not enough charge to start the car after setting overnight. I have replaced the battery and alternator and checked the fuse boxes. I was told it may be the power door locks or the on board computer. Anyone that has experienced this or has a suggestion, please reply.
Thank you.
Brian