I drive a 1999 tacoma TRD taco. It has been one great truck off roading with the locker. On the last attempt to engage the locker the light just kept blinking and the locker did not engage. Has anybody out there ever had this problem? Where do I start looking to fix this problem. Can anybody help with this one?? Thanks

Disconnect and reconnect the plug for the actuator power. Clean the contacts if necessary. Might just be a loose or dirty connection.

Could also be a bad L4 switch or bad vehicle speed sensor.

I tried the plug allready, where is the L4 switch located and where is the speed sensor. Are these dealer install items only?

Both the L4 switch and the speed sensor are around the top of the transfer case. In your case, I don't know if you can remove them from the transfer case without removing the transfer case from the truck. You should be able to test the L4 switch for continuity if you can access the plug. The speedometer should be an indicator of whether the vehicle speed sensor is working correctly or not.