When you press the brakes and the emergency flasher circuit is active, ALL the lights light up. Rear lamps, front lamps, dash lamps.
Do your emergency flashers work OK? It could be that the flasher circuit is shorted out or energized yet the flasher isn't working. Then EVERYTHING lights up when you hit the brakes. Try turning the emergency flashers on and then off a couple times by pushing the button in and pulling it out - in case it's stuck or sticking (?).
Check and see if the front signal lamps light up when you hit the brakes. That would be indicative of the emergency flasher circuit tying all the lamps together. If they ALL light up when you hit the brakes, check your emergency circuit flasher. It could be malfunctioning (or missing). I had a 1970 GTO where the emergency flasher was missing and the flasher button was pushed in that acted like yours does. No turn signals, only ALL the lamps when I hit the brakes. Took me a while to figure it out.....
If the dash lamps light up, or all the lamps light up but the emergency flasher circuit is working right, you'll either need the mentioned adaptor or get to swap out the turn-signal switch. Great fun if it's the switch.
Good luck!
