When these dual filament bulbs go bad, one filament drops down because it breaks in the center. It shorts over to the other filament, causing an unwanted connection between circuits. Then all sorts of thing happen, like brake lights cause blinker bulbs to light and blinkers flash quickly because extra bulbs are added to the circuit.

Since time is money, just pull all those 40 year old bulbs out and put in fresh ones. That way, they don't embarss you on the way to the cruise. When you install them, add a bit of dialectric grease to the contacts and the base to keep moisture out of the socket and to keep the bulb from seizing in the holder.

Amervo, I noticed all the MOFO lamps were working on the way over to the show. Maybe we have no brakes, no heat or are up on jacks. But our lamps shine bright and proud.