The brake lights,head lights,and running lights work sometimes and don't other times. What might be my problem?

the 1/0 black battery cable is making a corroded connection to the chassis...this is an important one as needs to pull at least 300 cold cranking amps...what you must do is disconnect the mounting point to the chassis and scrub this big hunkin 1/0 black battery cable at the end where the lug is...wirebrush the ground spot to insure a 300 amp connection...your tail lights and ignition and dash are on this ground as well and if enough corrosion is there I am quite surprised there is any electrical connection at all there....also get a BRAND NEW battery as these new tin/antimony (NO LEAD) battteries will last only five years before they do not want to charge up anymore....playin' hell with your alternator~

I would follow the above first but - getting the new battery yet.Check your voltage from the battery with the car not running . Check with Negative post not connected make sure you have at least 12 volts then hook up the negative post and check at the battery and see if you have at least 12 volts.Then while checking for 12 volts on the battery move your negative test lead to ground points on the car while keeping the positive lead on the battery.The negative lead should give you 12 volts just about everywhere on the cars metal surface,engine block,fender walls ,Firewall,and so on.You must start at the basics .Just don't worry if you see a little drop in voltage when you hook the negative post back up there are things being drawn from the battery while the car is not running but are very minner.Start your car and do the same checks but do not take of the negative post while the car is running.you should have when you start the car 13 or more volt and this will tell you if you battery is getting charged,It will not tell you the battery is holding the charge.But will let you know the charging system is most likely working.Look around for any ground straps they are at the back of the engine to the firewall ,and other locations.Make sure of the obvious corrosion issues and the like.The thing that I hope is not the issue is the LCM- Lighting Control Module but that should be the last thing to look at .it is under the dash above the gas peddle but this is something you might want to have someone check out for any codes before you dive into that. Have a scan done of the on board CPU for trouble codes before you go any further.

The typical failure for this year model is the LCM. It handles just about every light in the car. I've changed out 2 in '06's and 3 in '08's just in the past 3 months. (We have a lot of Crown Victorias) Same car, different badges - the Mercury is prettier.

My failed LCM never gave me any trouble codes when it took the headlights out. If one of your symptoms is that the high and low beams do not come on, yet the lights do energize when you hold the "flash to pass" lever towards you, then it probably IS the LCM... and there is now VERY GOOD NEWS!!! Ford just started fixing them for FREE!!! (Ref: Ford Customer Satisfaction Program 14N01) Ford/Lincoln/Mercury has finally decided to deal with the rat-nasty LCM's they have been putting out. I just got the letter on this from Ford last week. Without starting a safety recall, (as they should,) they are extending the warranty on the LCM (based upon VIN number,) to 15 years or 250,000 miles whichever comes first. AND if you paid earlier for this replacement and have the receipt, they will REIMBURSE you for the repair provided it was done before the notice came out. The window to be reimbursed for an earlier repair looks to be between April 1st, 2014 to October 31st, 2014. This window also applies if your car is under 15 years, yet has exceeded 250,000 miles. Ford's LCM replacement is completely without charge for parts and labor. Retail owners should have their VIN number handy and call the local Ford/Lincoln/Mercury dealership for an appointment. They say it should take a half a day yet could be longer due to dealer "service scheduling requirements." They will only replace LCM's that do not work, but mine lasted 10 years before it started acting up. This gives Ford a way to show concern, without admitting fault, or really dealing with the 1000's of LCM's that will fail after this window closes in the future. Ford only "invented" their LCM after their service departments were losing money on a system that smaller repair shops could easily fix. Nice to have a little black box that they could unplug and swap out so they could charge $800-$1000 in parts and labor then repair at the factory with three $10 relays. They used to mount these relays near or on the fuse-box where even an owner could replace them if so inclined for between $8-10 each. Anyway if you have any questions about this "un- recall" you may contact the Ford Customer Relationship (I'm in a "relationship?") Center at 1 (866) 436-7332 either they or the dealer can tell if your vehicle is covered if you have the VIN number handy. IN THE MEANTIME. Should you need to get granny to the hospital at night, you can try this AT YOUR OWN RISK. The LCM on my 2003 is located about 18" directly above the gas pedal under the back of the dash... Turn on the lights and give the LCM a tap with your foot. This will often engage the cheap Ford relays and the lights will come on. But you should make that appointment with the dealer before everybody starts fighting for access to the same part. I also don't know if Ford would be willing to call someone a liar if he claimed his headlights stopped working because he thought it is just a matter of time before the lousy relays in his LCM failed, since this is intermittent, and asked for an LCM replacement under this warranty as well. That is why this should be a RECALL, and not just some bean- counter's way of slithering out from all of the angry customers that demand action today. My Grand Marquis is really a fantastic car, but having to drive 50 miles home on narrow country roads with one terrified hand while holding the "flash to pass" arm with the other, and risking my life because Ford played it cheap with some relays on their flagship chassis for a decade or more, really makes you wonder what they were thinking.Remember: you have to take your car to a DEALERSHIP to get this fixed for free under warranty. DO NOT TAKE IT TO A PRIVATE REPAIR SHOP unless you just want to throw your money away!

Thanks