Here is all you really need to know:

The switchback bulbs are very simple.  They light white when there is voltage applied across the + dim/low/white/running light circuit.  They light amber when there is voltage applied across the + bright/bright/amber/turn signal circuit.

Basic Diagnostics:

1.  If your switch back bulbs are lighting up white, then there is voltage being applied to the + white contacts of the bulb by the system.

2.  If your switch back bulbs are lighting up amber, then there is voltage being applied to the + amber contacts of the bulb by the system.

3.  If your switch back bulbs are not lighting up, then there is no voltage being applied to either the + amber contacts or the + white contacts of the bulb by the system.

Basic Fix:

1.  To make the switchback bulb light up white, apply 12 volts to the + white contact.

2.  To make the switchback bulb light up amber, apply 12 volts to the + amber contact.  

3.  To make the switchback bulb not light up, reduce voltage to less than 12 volts across the + amber and + white contacts.

B.  The led bulb contacts may not be making solid electric contact with metal contacts in the socket.  Check the contacts on the bulbs and socket.  Realign or adjust as needed.  Reseat the bulb.

C.  Are you sure the filament bulb still works?  Often in the process of installing, you can pop a fuse or lose a wire/connection to the socket.  Always use the glass bulb or a voltage meter to verify there is at least +12 volts to the socket. 

D.  In some vehicles some automotive circuits will not fully energize the circuit, or pulse current to the bulbs to check to see if the bulb is burned out.  Since the leds draw 90% less power than the glass bulbs, these circuits do not switch on full voltage , or will not stay on when the led bulb is inserted.  This can be corrected by installing one 6 ohm load equalizer across the circuit at each led bulb.  This will add more load to the circuit and make the full 12 volt power switch on.   In this instance, the glass bulb will make the power switch on, but the led bulb by itself will not make the full 12 volts switch on.  

This creates the illusion that the led bulb is at fault when in fact there is not 12 volts at the connection when the led bulb is inserted.

B.  The led bulb contacts may not be making solid electric contact with metal contacts in the socket.  Check the contacts on the bulbs and socket.  Realign or adjust as needed.  Reseat the bulb.

C.  Are you sure the filament bulb still works?  Often in the process of installing, you can pop a fuse or lose a wire/connection to the socket.  Always use the glass bulb or a voltage meter to verify there is at least +12 volts to the socket. 

D.  In some vehicles some automotive circuits will not fully energize the circuit, or pulse current to the bulbs to check to see if the bulb is burned out.  Since the leds draw 90% less power than the glass bulbs, these circuits do not switch on full voltage , or will not stay on when the led bulb is inserted.  This can be corrected by installing one 6 ohm load equalizer across the circuit at each led bulb.  This will add more load to the circuit and make the full 12 volt power switch on.   In this instance, the glass bulb will make the power switch on, but the led bulb by itself will not make the full 12 volts switch on.  

This creates the illusion that the led bulb is at fault when in fact there is not 12 volts at the connection when the led bulb is inserted.

With many flashers, led bulbs will cause the turns signals to flash rapidly, or not flash at all.  

If using led bulbs for turn signal applications, or in vehicles with bulb-out warning systems, like Mercedes, BMW and others, you may need to add load equalizers.    

For led turn signals, you can change the flasher out using one of our new patented combination led/oem bulb flashers.  Most electronic flashers are not the same as an led flasher.   Many electronic flashers do not necessarily work with led bulbs, unless they are led flashers.

If we do not have an led flasher that matches your flasher, use two 6 ohm equalizers for two front, or two rear led turn signal bulbs.

Place one load equalizer at each led turn signal bulb location.    For turn signal led bulbs, the equalizer must be spliced across the - ground wire and the + turn signal wire feeding each led bulb.

More details on load equalizers and led flashers can be found at:    

http://superbrightbulbs.com/equalizers.htm

A. You may be using a 3157 switchback bulb in a 3156 socket.  Check to see how many wires there are going to the bulb socket.  If there are only two wires going to the socket, then it is a 3156 socket and the 3157 bulbs will not work in a 3156 socket.  

B.  There is voltage being applied to the + amber contacts of the bulb by the system.  Determine the source and correct as needed.

The Amber -White (AW) Switchback bulb has special circuitry inside that senses when there is voltage on the (amber) turn signal circuit.  Whenever there is voltage on the (amber) turn signal circuit, the bulb switches to amber mode.  The result is that in most vehicles, the AW switchback will flash amber-off-amber off whenever the turn signal is activated and the headlights/running lights are off and amber-white-amber-white whenever the turn signal is activated and the headlights are on. the switch back bulb will flash amber-white-amber-white,  whenever the turn signal is activated and the headlights are on. 

In some vehicles, the power to the (white) running light circuit is automatically switched off whenever the turn signal is activated.  In these cases, the AW switchback will flash amber-off-amber off whenever the turn signal is activated.