By tracing I meant to get an extra lamp with two long wires(control lamp), turn signal 'on' and watch front lamp blink, connect one wire from control lamp to the engine/ground, and with other 'free' wire first connect to relay - your control lamp should be blinking (so you know it works). Now, following the electrical diagram keep poking free wire at every connection spot where signal travels from relay to rear lamp. At some point control lamp will no longer blink - that will tell you where the problem is.