What's the little bulb in the tail lamp fixture used for?

In my experiences, the smaller bulb is meant to be the license plate light.


 I am not sure of your larger picture. Is this the left-side light, and is the right set up the same?

The light coming on with ground disconnected illustrates a ground problem... i.e. the fixture isn't grounded when unmounted, so power grounds back through the smaller bulb.


It wouldn't make sense if the license is placed in the middle, except for the fact the manufacturer may have offered only the pairs with the "extra" bulb for one side. If your plate mounts below this lamp then my guess is that is what it is intended for.

Ten's answer is a good one,but I happen to know your trailer is wired with a 7 way plug as is your TV. What he said on the grounding out though.

I looks like the black wire runs to the other side tail lamp,if it stops at the plate lamp on the way,its your running lights and would go on to the markers as well.

Probably the white one goes to the other side as well,but does not go to the plate lamp. This would be your brake lights.

The green wire probably goes direct to the tow plug. This would be your turn signal.

If you eliminate the small bulb,you will not have brake lights or running lights depending on how its wired (colors above) without also a working multi meter,i'll skip testing details..

 Rotten ,Nasty dried up cork gaskets..

Thank you for all the responses.

The larger bulb is a duel filament.

New pigtails have been ordered for both sockets in both fixtures.

I was on this thing yesterday and having so much fun.... and closed up shop before checking wires at the fixture.

Cheers,

Rod

I did some digging in the Avalon today. It has a pair of Auto Lamp 575's on it. They are set up the same way, but are surface-mounted.


I couldn't get the lights functioning correctly, for some reason am losing the ground, even though they had worked to tow it home. 
Tracing the wiring back is fairly easy in this trailer because it has the trunk-back, and the wiring runs through it in the open.

Left-side wiring:


It appears that the green wire runs the marker light circuit, and runs to the double-filament bulb.   The red wire also traces to the double filament and lights up both sides when hot.  I am believing that the brake and running lights wire to the larger bulb, and the smaller bulbs are supposed to function as the turn signals.  In older vehicles, the brake lights were wired as a stand-alone circuit, where the modern wiring lights up both sides by powering through the turn-signal wiring simultaneously.  These are different lights than yours but I think there is a good chance they're built to function the same.  The only real way to know for sure is to ground everything and ground it again, then trace each circuit individually with a power source to see what lights up what....

As 10 pointed out the small bulb is usually for a plate lamp,probably since about 1960's. just to keep things clear..

In your case 1952 and those particular lights,the small bulbs are for turn signals,or whatever the installer wired them for, just as 10's Avalon lights.

I have dual bulb bargman 99s and from searching the net, I came across an old diagram that showed 3 different tow vehicle-trailer wire setups.
So the small bulb is a taillight bulb. The bigger dual filament is the turn/stop bulb. Back then, it was a 1034 (for 12v). An 1157 is an acceptable replacement.

One setup was just a single bulb using a 1034 as the tail and turn/stop. The turn signal overrides the brake light function.

Second was a dual bulb setup with the small tail bulb and the larger 1034 as the turn and stop. The turn function still overrides the stop function.

Last setup was dual bulb and all functions were independent of each other.

The dual bulb setups had an extra wire going to the tow vehicle and I think this confuses many who are more familiar with single bulb setups and that extra wire becomes a mystery. I don't know why there were different setups, but maybe from the old 6v days.

I don't use the dual bulb setup. An 1157 is what I'm doing now. If I was going to use the extra socket again, I would use it as an additional brake light.