Check engine light on 2003 ford expedition - 2003 Ford Expedition

SOURCE: 2003 Ford Expedition radio will not work

whats the radio serial number

SOURCE: 2003 ford expedition

P0300 Random/Multiple Cylinder Misfire Detected
See all 300-series codes here: http://www.ford-trucks.com/article/idx/7/241/article/Ford_OBDII_Diagnostic_Trouble_Codes__P0300P0399__Ignition_System_or_Misfire.html

SOURCE: 2003 ford expedition "door ajar" & interior lights staying on

the wire on the door latch make sure they are plugged in or unplug it and the light should go out if it does not then it's not that door

SOURCE: how to replace the light bulb on exterior mirror 2003 ford expedition

The one with the turnsignal intergrated into it?

If so.....

Push the Mirror FORWARD, up close to the pivot poit there is a small torx head screw - unscrew that.
At the tapered end carefully pull/pry down. That end has a small metal spring clip that holds it up, be careful with it, if it snaps, you can NOT by a new one...

SOURCE: Ford 2003 Expedition EB. Check Charging System

Its probably the alternator. It will need to be tested on the vehicle with the engine running and the light illuminated. Measure the battery voltage with the engine running. It should be at least 13.5 volts. If it is lower have the alternator tested at a shop with a charging system tester.