I have a Ford Windstar 1999 . My license plate lights are both out. 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 lamps on the fuse circuit work fine. When lamps are lit, shaking the wiring harness has no effect. Can one explain to me what is happening and where? This is very strange!
What does shaking the wires do when the switch is on and the lights are off? I suspect the ground is weak in the rear of this van. Try a set of jumper cabler end to end between the negative battery post snd the ground wire for the lights.
What a coincidence, I posted the same question on same car yesterday! See link and text below:
“License plate lights on Windstar 1999”
http://community.cartalk.com/discussion/2281396/license-plate-lights-on-windstar-1999
“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.”
UPDATE
When I did ground the ground wire of my license light to the battery instead of its normal connection to the rear ECM, then the lights are ON all the time (even if lights are OFF at the dashboard). That suggested that the license lights are controlled by a ground switch on the ECM and that the (+) wire is always hot. It also reinforced the bad feeling that the issue is on the rear electronic module! And since I refuse to change it for a license light issue (nope, not even for a used one!) it got us thinking of another solution. I say “us” because I took the car to my trusted Ford dealer this am, and the “fix” of our choice was to splice the ground wire of the license lights into the ground wire of the rear parking lights…et voila! It appears that the dealer tech was impressed by my ground work (pun intended) and did the fix free of charge… and that is a super friendly dealership!!
It is true that I would still want to understand what exactly is happening on this defective circuit board… definitely worth thinking about a little more.
Physics was not exactly my major. Could it be that the different effect of having the licence lights connected after or before switching the dashboard reflects that there may still be a weak gound on the board and if one switches the dash ON it enables charged particules to accumulate at the interrupted license ends, and that may result in a voltage high enough after the connection Whereas when switching the dash on a (pseudo) complete circuit the difference of potential at license light poles never gets high enough???
The ground was the problem. Apparently bad rear light control module. The fix was to re-wire the ground connection around the rear light control module in the tire jack bay to avoid the very high cost of new module and associated programming.