Rear light question 1968 GT350

[mustangman-nl]

The rear lights on my GT350 aren't working well:

When I have the driving lights on, they are very bright
When I have the driving lights off, the left or right sequential lights are working well and are bright as well
When I have the driving lights on, the left or right sequential lights are hard to see working

Were too start searching, it appears that the ground is good, can it be the dynamite sticks?

Thanks and regards,

Reiny

[69 GT350 Vert]

I would start with looking at the turn signal switch.