[Charlie Brengun]

This is on a '68 GTO. I changed the hazard and turn signal flashers, made sure the front and rear earth for the lights are good.
The turn signal produces a steady bright light but it does not flash. It's just on or off.

I think there must be a problem with the earth for the turn signal switch, just can't get my head around how it get it's earth from the steering column (attached picture).
Or do I need just need to install the steering wheel before testing this again?

[OG68]

The signal 'switch' doesn't have an earth. It just transfers voltage from the battery to the bulbs.

There is a flasher canister mounted on the lower dash to the left of the steering column. It has a bi-metallic reed that heats up when current is passed through and opens the 12 volt power source to the switch. After the reed cools down it closes the circuit and starts the process over again. It may need replacement or is of the wrong type.

There is a second canister mounted at the top of the fuse box. It is used for the hazard flashers and works on the same principle as the signal flasher.
The type of canister is dependent on how many bulbs are in the circuit. A heavy duty canister with a larger internal reed (higher bulb current to open the circuit) would be used for the hazard lights and for trailer towing. Used for a signal application, it may never get hot enough to open the circuit.

LED bulbs may also cause your problem. Since they are an electronic device and draw minimal current, there isn't enough current to open the canister reed switch. You will need an electric signal canister designed for LED bulbs.

[Charlie Brengun]

Problem is the led lights I am using. I did use led flashers, even a couple different types but that didn't help.
This evening I wired in a pair of relays for the front turn signals and that fixed the fronts including the hazard lights.
Now I just need to wire in relays for the the rears.