First thing I would do is check ALL the bulbs. I know that sounds silly but you might just get lucky.ðŸ›
You are right. I got this problem before a time or three.
What happened is sometimes the bulb circuit touch inside and power flow through the other circuit this way. Try removing one bulb at a time and see. Might be corrosion behind a bulb too, or touching connection. Try to work systematically, you'll find the problem.
For the brake light, I also suspect the turn signal switch. Take it appart and look inside, might just need a clean up of the contacts.
Good luck!
J. F.
