Since it appears to be speed and/or motion related, I'd look at places that get pinched or move. Because it isn't replicated by using the brakes when not moving, I wouldn't place all the eggs in the brake circuit basket, but you've got to start somewhere.

I'd trace back from the brake lights. Look for where the harness goes under the sheet metal panel near the fuel tank. Look for where it passes under the seat. If there's a place where it flexes, maybe it's chafed inside.

Or start at the fuse box and go downstream.

Carpet and seats are easy to remove.