1995 Ford Thunderbird left rear brake/signal light

Hi Mike. Welcome to the forum. First have a helper press the brake pedal while you watch the rear lights. If the failed one goes out while the right side turns on, the bad side has a bad ground. That could be corrosion in the socket, a broken wire, or a corroded splice for all of the sockets' ground wires. Ford likes to use a lot of wires tied together in one terminal that is bolted to the body sheet metal. That one should be ok since some of the bulbs are working.

Next, a good place to start is by removing all of the rear working bulbs from their sockets and look at just the bad one. If that bulb never turns on now, again, suspect a bad ground. Weird symptoms are caused by current going through the filaments of the bulbs on the other side when one socket has a bad ground. That's why removing the good-working bulbs can change the symptoms of the bad one.

If the bad bulb never lights up with the head lights off and the brake pedal pressed, look closely at the contacts in the socket. If your bulbs have the flat plastic bases, corrosion on the socket terminals is fairly common. Often they can be shined up with sandpaper. If your car still uses the bulbs with round brass bases, a poor connection can cause heat buildup that melts the round plastic disc holding the contacts. Pressing the bulb into the socket may identify that problem. The fix is to either replace the socket, or add a small bead of solder to the affected contact on the base of the bulb so it can reach down to the contact in the socket.

If you don't find anything obvious, you'll need to use a test light or an inexpensive digital voltmeter to see if there is voltage present to that socket. With the round brass base, one terminal will have 12 volts with the head lights turned on. The other one will have 12 volts with the brake pedal pressed or when the signals flash on. If you have two signal / brake lights per side and one of them works, you know the wiring from the turn signal switch to the rear is ok. If you only have one bulb per side, and voltage is missing to just the left brake / signal terminal, there could be a break in that wire all the way to the switch, or there could be a defective contact in the switch itself.

Caradiodoc

Just so we don't over look something silly, you are aware there are two different filaments in each bulb, right? It would appear the left tail light filament is working fine and the brake / signal filament is not.

Forget the melted contact part of my earlier story. That doesn't pertain to your type of socket. The place to start is by probing the socket with a test light. There are four contacts arranged like this, (the periods are there to get the spacing right):

.A.B
____ ____
____ ____

.C.D

A and C are tied together and are the ground contacts, one for each filament. Between B and D, one of them should have 12 volts when the head lights / tail lights are on. It's the other one you're after. It should have 12 volts when the brake lights are on. If it is missing, you'll have to follow the wire back, possibly as far as the signal switch, but before you start ripping stuff apart under the carpet, the next most logical place to find the problem will be in the switch itself. If voltage is missing on the wire there, the switch is the problem. If you find voltage there, the wire has a break in it.

There will be three wires going to the socket. On some sockets you can back-probe them through the rubber seal. I don't like piercing the insulation on the wires to take a measurement because that will let moisture in and lead to corrosion. The wire you're after is light green with an orange stripe. Check for 12 volts on him when the brake pedal is pressed. If it is missing, the next step will be to remove the covers from the steering column so you can access the connector on the signal switch. Look for the same color wire there and check it for 12 volts when the pedal is pressed.

Caradiodoc