help with lights

Hi there before i start i want to said i m new to this cars
i try to do the staff my self but i run in to problems , i did some search in here
and i can find something similar to my problem .
i got a 1950 desoto 2 doors custom
the problem is that when my head lights and my tail lights are on there is no brake or turn signals in the back but if my head lights are off there is brake and turn signals . i check my bulbs and they are ok, i change them to see if
maybe that's what it was but still the same .
the lights on the back are bey bright and when they are on the other filament
is is very light . can i adapt a extras light in there?
thanks for the help

 

did a `50 Desoto have turn signals from the factory , or was it added on? if they were added on i'm guessing it wasn't wired correctly

 

The tailight assemblies MUST be GROUNDED well for them to work correctly. Right now each filament is using the other filament (in same bulb) to return current to ground. One way is dim, the other way brighter. But if you get a good ground, both tail and stop/turn will be brighter than now.

 

I have to agree you have poor or no ground to the lights. Check it to the tail lights, head lights and front turns also.