Brake light gone funky

[spiderdude]

I have an old w reg KA that serves as a workhorse mainly. Drivers side brake light went a few days ago and a mate slipped a spare one he had in to replace it. He didn't realise it was a double filament one he put in replacing a single. When I drove it and indicated, the indicator would stop when I pressed the brake and come back on when I stopped. I checked and replaced the bulb for the right one but it's still doing it. Plus when I brake all 3 lights on the cluster come on and the brake light is dull as anything. Any ideas as to what the most likely cause is would be appreciated.

[supraking]

sounds like a crossed wire/connection some where , pull it all out and check everything off the car