What can cause this? Its a 2000 supercrew 5.4 4x4

 

Alternator. Go to the parts store and get the battery and alternator tested.

 

Do what glc said and also check your connections......

 

Check your alternator fuse. The inline fuse.

 

For sure, along with the checking of alternator and battery.

 

checked alternator and battery they seem fine the guy at advance auto said its a drian some where possibly? what could be draining the system.

 

A bad connection. Check that loop down by the starter, - sometimes they hold water. You have to go thru your cable connections/grounds.

Keep in mind that MOST of the time and with light illumination, it is an alternator fault. Doesn't have to be of course, in my experience it has been.

So after going thru it and if the problem still exists, -look closer at the Alt.

 

I'm thinking alternator.