I have a Ford Windstar 1999 (80K miles). Reliable so far (for 7 +years). My license plate lights are both out. I grab a voltmeter and find no difference in voltage between the 2 poles of the light(s) and indeed a new bulb does not help. I remove the inside panel of the back door, there is a connector between the license lamps and the rest of the car electrical system. I disconnect it. I go turn on the lamps at the dash board, I then reconnect the connector to the license plate lamps at the rear door… and they work! I go back to the dash and switch the lamps (controls side +rear lamps and license) OFF and then immediately back ON, and the license plate lamps stay OUT. All the others side lamps work fine. Can one explain to me what is happening and where? likely there is no grounding issue at the license plate lamps since I did the same test using a small tester lamp I assembled and plugged into the connector at the rear door.
You have the right idea, using a voltmeter and checking for +12v where it is supposed to be. Now you need a helper… someone to just work the switch on/off at your command while you continue to check for voltage as you wiggle wires. Likely there is a wire break or a weak connection somewhere.
You must also double check for a good ground. If uncertain, make your own ground and test. I had a similar problem on an old pickup truck. The bulb sockets were not making a good ground connection.
Thanks Steve, I had not tried substituting just the ground , and if I do that (connect the ground of bulb to the battery (-), while the (+) of bulb is connected to the rear electronic module… then the licence lamp is ON all the time (with all lights OFF)… and any ON/OFF action after that. I am confused… and probably headed for the shop in the am.
I also know it is not a bulb socket issue because I have used as a tester a brand new bulb+socket +decently crimped connectors I just made.