Just put new battery in and two weeks latter its dead again so now two new batteries and car is dead again alter and stater good 2000 Marquis

Some of the guys at parts houses know what they are doing, and very well educated in auto electrical systems. The some of them are not. Did the test the voltage regulator? An alternator can test 'good' but if the regulator is bad, all bets are off. Have a look here: http://troubleshootmyvehicle.com/ford/4.6L-5.4L/how-to-test-the-alternator-2

No they did not check the regulator. Thanks I will test and see. Also the employee would not give me the print out of the test just said its good. I wish people would just be honest, if you don't know or know how speak up. I'm glad I have learn to listen to my car so I haven't ended up on the side of the road in a long time. Thanks again I will keep posting.

He probably didn't know how to interpret the read-out and didn't want to admit it, so won't show you to avoid questions....

Something could be draining the battery, like a trunk light that doesn't go off. Hook up an amp tester and full fuses one by one. Also, and alternator can charge well, but still have bad diodes and drain the battery. Lots of people have similar problems, and they can be hard to find. Disconnect the battery, and measure how many amps or milliamps the car draws with doors closed and everything off. Anything over 1/2 amp, and something is wrong.