Why is battery draining ford 2002 Taurus SES

Check engine light has been on for several years, had it cleared once but came back on but still ran fine. Over the last year have had passenger floor board wet after heavy rain. Over last several months odd things happening with warning lights especially after it rains. Shows doors open kept locking and unlocking door after stopping 3 times and open close all doors and trunk finally works and lights go out. This has happened a few times now. Emergency brake light on even though not engaged. Would go out then back on then finally went out and stayed out. Oil light will come on, check oil is fine. Then light goes back off. Now gas cap light came on even though gas cap is locked. Gas cap light has stayed on. About 3 months ago, battery dead. Jumped, started right up. A week later, dead again, jumped, had battery checked. Tested fine, took to mechanic explained what was happening they said alternator bad. They ordered one was not the right one, ordered a 2nd one also not the right one. Mechanic sent picture and finally received the one that mounted the same as mine. Evidently there are 3 listed for my car and mine was the most expensive. Mechanic said had something to do with voltage? $400 for part and labor. Ran fine for almost 2 months then dead again, jumped and 3 days later dead again. Back to mechanic they said alternator bad and replaced again at no charge. Not even 2 weeks later dead again. Had not driven for approx. 5 days took longer to jump but did finally start. Need to go back to mechanic. I still think has to be electrical prob as something keeps draining battery. Need to go back to the mechanic. Any ideas? I feel like I am doing research for the mechanic but need to get this corrected. Car has 95,000 miles.