I have two separate wires, that I put in, that grounds each tail light housing and the socket them selves, those wires are screwed into the body using a star washer. I painted over the screw and washer to tidy it up..... haven't had an issue since.

 

So I just have to run a wire from each of the taillight housings to the body? I should have mentioned that the problem I'm having is the parking lights work but the when I step on the brake or turn on the blinker not all 4 bulbs light up in the taillights. It varies from time to time but one or two of them are always not working.

 

The 1157 bulbs used in those taillights have two filaments. One is used for the park/tail lights. The other is used for the brake/turn signals. The wiring for these two circuits is completely separate EXCEPT for the ground at each bulb. Unfortunately, a bad (or worse, intermittently bad) ground can lead to weird problems. The first thing you really need to determine is WHICH filament is illuminating under what conditions. For example, a bad ground can cause the tail light circuit to ground through the turn signal filament, causing both to come on. When you step on the brakes, this ground is lost and both go out. If you can remove the lens and look at the bulb while testing this, it makes proper diagnosis easier.

 

tail light sockets have been a horrible issue..

poor ground connections.. i have taken usually to putting a noose of 18 gauge wire around as shown.. twisting it tight.. shoving the wire thru a hole in the socket.. and grounding it..

the best looking replacement sockets i saw came from classic industries. i don't know what brand they are..

many of the replacements are COPIES without engineering them to work.. or understanding how they work.. and how they clip into the reflector..

the Pico electric looks decent but i have not had one in my hands yet.. #5403C (#5403PT)
3-Wire Twist Lock Stop-Tail-Turn-Park Dbl Contact
GM 1969+ #8909908 & 8903283

Pico No 5403 | 3 Wire Park / Stop / Tail / Turn | Pico of Canada Ltd.

this might also work.. Pico No 5404 | 2 Wire Park / Stop / Tail / Turn | Pico of Canada Ltd.

this is the one from OPGI..

https://www.opgi.com/el-camino/CH28290/

i don't know if it will fit your housings.. and i have not had one in my hand.. see the images below to note the too short latching straps that just were not engineered properly..

i had to do the loop the wire on the tail light bulbs in my long gone 66 GTO..

 

I "fixed" the tail light problem in my '56 wagon once and for all. I replaced the cheap, push in style sockets with later model twist in sockets and ground the housing to the car body.
One of the cheap bulb sockets. Note there are only two prongs left. There "were" six. One was gone when I removed the lens. I pulled the bulb to check it and broke three off trying to re-install it. The other broke off as I was trying to figure out how to get the socket to stay in the housing...

Some of the wiring... drivers side. Passenger side was identical. At least they were consistent.

NOW! This is what I'm thinking. Late model bulb socket. Modify the '56 housing to accept the new style twist in.

Because these are GM the sockets even have the correct color leads coming out of them. Yellow = left, Green = right, brown = tail lights and the black is ground for the socket.

As installed.

Made a rubbing of the housing the lights came from. Not a lot to remove here. A little work with a Dremel and a carbide bit and the new sockets will drop right in...

It took about a half hour to get things set up, make a "good" template and throw some Dykem on the housings.
Here is the good template. Made out of heavy card stock centered to the lens mounting holes and located with some aluminum rod in the bolt holes.

Used a Dremel with a fiber blade to block out the notches. Hogged out most of the material with a small carbide and then sweetened up the slots and oversized hole with a small grinding stone.
First test fit. The hole is still kinda rough.

After a little more fitting and a bit of a hair cut to get the tabs to slip over the shoulder of the new hole.

The new socket in place.

This went pretty quick, about two hours start to finish for both housings.
Mark

 

Theres a phillips head in my console so I can jiggle bulbs before heading out. What a pain, I feel your frustration.

I plan to modify my 69 GP tail light housings and use modern sockets with a tail light harness from the salvage yard. Our cars are similar but mine is kind of tight on space between the lamp and body. I bet you can come up with something much more reliable by adapting your housings to the more modern equivalent of your 1157s with appropriate sockets implanted. Wiring not my specialty though! I believe the sockets are the problem and replacements are loosy-goosy holding the bulbs. Of course there are probably LED tails for yours available by now if you're into that look.

 

You might have bad sockets, but your main problem is a bad ground. Run a 16 gauge wire, 14 is almost overkill, from the grounding point on the body through the floor and to the frame. Make sure all points are bright and shiny and tight and use star washers.

 

Additionally, for future use, here's a general document on grounding, originally attributed to Ron Francis Wiring, (possibly modified); see attachment:

 

Good looking conversion. Easy simple fix is, new bayonet sockets are cheap, add a ground wire from the tail light housing to the body, no need to go to the frame.

 

On not needing to go to the frame quite possibly, but why not eliminate the possibility of a poor ground? Bodies are bolted to the frame and over the years rust can and does accumulate at those points although not as bad as early pickup truck beds. In the 60's and through to the 80's or so it wasn't uncommon to see a pickup with the tail lights flashing on and off as it went down the road and at least 95% of the time it was due to poor grounds, I fixed many by merely running a jumper from the bed to the frame. If it were mine that's how I'd fix it to help eliminate getting rear ended at night because of poorly operating tail lights.

 

Not a fan of drilling a hole in a sealed compartment to run a ground wire thru. If the interior cabin components are working, via a good body ground, one can assume the rest of the body (tail panel) is also grounded. IMO attaching a new ground wire to the tail light mount via an existing screw/bolt, is a better option.

 

I'm not a fan of assuming anything especially when it comes to safety. I learned along time ago working on commercial and military aircraft at McDonnell Douglas after I was discharged from the Marine Corps that the real meaning of "assume" is that it makes an a.. of you and me.

 

OK it's a fact, unless the tail panel is not connected to the car, if the interior items work, the ground potential to rest of the body is constant.