Sounds like a bad ground, probably in the ligh assembly but might try R&Ring the connector.

 

Have you just changed a bulb?

The wrong bulb can cause odd things to happen to the lights.

Does the turn signal flash at the same time as the brake lights?

 

 

 

What is the bulb number you put in?
The wrong bulb makes contact with the socket contacts that it should not touch and causes problems like this.

 

I believe our tail light bulbs have elongated contacts rather than the standard round ones. They must have done that for a reason?

 

 

I'm going to say the same things others have said , just in a different way. Remove the bulbs from that harness you disconnect. Look at where the bulb makes contact with the metal prongs. Is it burnt? Is there any indication of melting or heat distortion to the socket. This is a known problem for our lights. What kind of bulb are you using in the front blinker...LED??