Why does the Door Ajar signal come on with all the doors closed?

This is typically an issue with a switch on one of the doors. Normally, it's a simple pin switch (much like you find on most refrigerators, that you can press to turn off the light). If the interior lights are not coming on when the "Door Ajar" indicator is illuminated, then I'd do a little deeper investigation. There is also a switch located inside the latch assembly on EACH door. There should be a wire connected to this switch. If it is disconnected it would cause the symptoms you are describing. The only way to check, though, is to pull the door panel off and visually inspect the connection. If any of the doors have been worked on lately check them first.