I just swapped out my 56's Shield Tail Lights for 42 Round Can's w/ glass.
The prior Shields worked fine with running, stop, brake and directionals. There were only two wires so I wired up the new cans and got only directionals. I swapped the wires around and got the same. I even tried the bulbs from the old. They have the same numbers with 12V marking and nothing but one element working on both, the directional.
I thought it was a simple swap, so what am my missing? Do I need to reset something?
Your help would be greatly appreciated.
Cheers !!!!

 

How many wires were used with the shields, 2? It is likely that the 42's were never made for directionals, and one of the wires is actually a ground. Look in the light bulb socket inside the lights, are there 2 contacts or 1?

 

I think Ross hit the nail on the head with it. The '42s would be set up for a 6v system and since turn signals weren't standard till '56 they probably only have one positive contact point inside. I'm not sure how the contact point housing (or whatever the thing the bulb goes into, lol) is on the '42, but if it only has the one contact point see if you can fit the dual-contact piece from the '56 in it. (two wires with brass contact ends and a plastic disk they sit in that holds them in place so the bulb touches it right)

This would be much easier for me to explain with pictures of what I'm talking about or actual nouns that properly convey the pieces, lol.

 

It was actually listed as a Ford 1932-35 style round, new tail light. They sell on eBay, LPM, MidFifties, etc. They are two wires, 12 volt, brand new. This one can be purchase with a Duolamp, but I choose not to. I heard many people swap them out on the 1953-56 F100's. They even make them with LED's because they say the red glass isn't that bright. It's a new light made to use two element bulbs, has two posts and should have all the functions.
I checked all the functions with the existing shield style 5 minutes before the swap on both sides. Now, what to do?

 

Check the ground. Without a separate ground wire the ground must pass through the mount back to the frame. If there is paint or tether light it may not act as a ground. Ground the light to a clean spot on the frame and check its operation.

 

Better yet, run a ground wire inside the housing and connect it to the base of the bulb holder. Run the grounds from the lights all the way back to something directly connected to the battery ground. It makes a tremendous difference. Remember, the bed itself is supported on rubber pads, where is it grounded?

 

Saw your post, thought I'd throw in my 2 cents, since my '46 runs those kinds of lights you want. The bucket is grounded to the bracket, which is grounded to the stake pocket, which is part of the bed, the final ground is where the fenders bolt to running boards. I know this when I removed my rear fenders and my lights stopped working. Put the fenders back and lights worked again.
Of the two wires from the bucket, one is stop light, one is running light. These lights were not originally equipped for turn signal. Hope this sheds a little light, no pun intended. The bulbs are dual contact, if 12V they are the old standard, 1157 bulbs

 

Lets see a tail light two wires one running lamp, one wire stop and turn
signal, in theory it should work fine, BUT if my memory serves me correct,
I think you should take a hard look at the bulb socket. Being original 6vt
bulbs #1156 I think and now you put 12vt 1157s,,, I really think the angle
of the contacts are different, thats why you get only one filliment lit. I think
you will have to replace the socket Napa for a #1157 bulb. So if the 6vt
and 12vt bulbs are compared I am sure the pins and contact angles are
slighty off. anyways I always liked the shields sam

 

Both 1156 & 1157 are 12 volt. One is a single element and one is a dual element, will not interchange.

 

Looks like I have some work ahead to diagnose the problem. I just thought that swapping one light with another of almost identical configuration (housings powercoated black, same numbered bulbs, etc.), there wouldn't be an issue. One thing I did notice is the older shield lights had nice brass posts (contacts) and springs in them. The new round lights have silvery, who knows contact and springs. And the new ones are not of the smoothest actions in the world. When putting in one bulb on one side, the whole socket pushed out the back of the housing.
Would running a wire directly from the socket to where? be a quick test to whether it is in fact a grounding issue?
Maybe pull that brass socket from the old lights and rig it to the new?
Thanks for all your help. Nothing's easy with this beauty.