Dual Switchbacks are cool park/turn signal bulbs to replace 3157 turn signals commonly found in most commercial vehicles today. This particular type of bulb is an Amber Off feature. This means that while you are running or parked, it is on White mode. When you are turning , then it is on the legal Amber mode.
Peterbilt 389 4 bulbs ( 6 months warranty )
Peterbilt 386/387 2 bulbs ( 6 months warranty )
Freightliner Cascadia 2 bulbs plus resistor ( no warranty )
Volvo 2 bulbs ( 6 months warranty )