I have a stock trailer, have tail lights but no brake or stop. My problem is that I can only find one wire going to this unit. Seems to me there has to be a wire for each filament. What am I missing here? Have checked for ground - appears to be good. Good signal at front of trailer, changed bulb and receptacle appears to be good and no corrosion evident. Wiring is mostly in conduit so it's hard to trace. Thanks for any help. Left turn only, right turn good.

The most common bulbs #1157 have 2 filaments and each has it's own Spring Loaded contact in the Socket. Remove the Bulb, using a 12 Volt Test Light to check that you have power to Tail light contact, then have someone apply the Brake and check the other terminal for 12 Volt power. Be careful to not short the probe to the socket housing! It may be possible that a wire has been severed somewhere between the front and rear of the trailer. Check for damage underneath the trailer or wherever the conduit is routed. Having driven 18 Wheelers, many trailers have their own fuses at the front of the trailer. Check wherever the cable that connects to the tow vehicle enters the trailer. Also check the terminals at the tow vehicle for power. Check individual fuses in the Tow Vehicle. I drive a 2004 Silverado and it has separate fuses for Trailer Left and Right Turn signals as well as for running lights. What kind of connector is used on the Trailer? ATA Trucking style or Bargman like an RV trailer. Remove the cover and check to see that there is a good connection to each pin. Is the Jacket on the Cable damaged and a wire severed? Anything between your trailer connector and the rear of the trailer is suspect. The most common bulbs #1157 have 2 filaments and each has it's own Spring Loaded contact in the Socket. Remove the Bulb, using a 12 Volt Test Light to check that you have power to Tail light contact, then have someone apply the Brake and check the other terminal for 12 Volt power. Be careful to not short the probe to the socket housing! It may be possible that a wire has been severed somewhere between the front and rear of the trailer. Check for damage underneath the trailer or wherever the conduit is routed. Having driven 18 Wheelers, many trailers have their own fuses at the front of the trailer. Check wherever the cable that connects to the tow vehicle enters the trailer. Also check the terminals at the tow vehicle for power. Check individual fuses in the Tow Vehicle. I drive a 2004 Silverado and it has separate fuses for Trailer Left and Right Turn signals as well as for running lights. What kind of connector is used on the Trailer? ATA Trucking style or Bargman like an RV trailer. Remove the cover and check to see that there is a good connection to each pin. Is the Jacket on the Cable damaged and a wire severed? Anything between your trailer connector and the rear of the trailer is suspect.