So I have an 03 ranger, 3.0 with 50k original miles, was driving fine last night, got up this morning tried to start it, wouldn’t even crank. It clicks very loud from inside the cab with the key out of the ignition. It only clicks with the door open though, once you close it, it stops, it also stops when you press the brake, and when you try and switch the headlights on. The red ALARM light also flashes continuously. When you try and start the truck everything just dies and doesn’t do anything. Once you stop and take the key out the clicking starts again.

 

Your battery cables on the battery need to be cleaned
Remove both and clean the terminals and the cable ends then reinstall

If problem is still there then disconnect Positive cable from battery, follow the smaller Positive cable to where it attaches to the Power distribution point, remove all cables from that post and clean then all then reinstall
Hook up positive battery cable and test if problem is still there