Alternator wiring help

[SKID]

I installed a projector battery monitor on tuesday, today the alam in it went off saying it was pumping 15.2 volts, I spoke to my auto elecy and he said the alternator was stuff so I bought a new one today, it is rated at putting out a max of 14.7 V, After a short drive when everything warmed up the alarm went off again saying it was putting out over 15 V. I call the sparky back on of the guys there said to check to make sure there wasnt a blown fuse, then he told me to pull the 2 wire plug out of the back of the alternator and put a test light on eash terminal in the plug, he said one should light the test light up bright the other should be dim, I did as he said one wire lights the light up the other doesnt, he said that was more than likely the sensor wire nd with out it any alternator would go crasy, Tested all fuses, all ok, when the car is realitivly cold to just warm the readings on the guage are fine, once it heats up, the voltage goes to 15 and over. called sparky again he said that a wire can be installed to bypass the break in the dead wire, rather than paying him can anyone tell me from the dead wire in the plug, where do I run the other end to?

its a 2000GU was origionally a 3ltr but previous owners converted it to a 4.2 so it a large degree the wiring under the bonnet has be barstardised, so I cant go by the wiring diagrams in my manual.....

Any advise one where I run the wire to fro the plug would be greatly appreicated...

Thanks in advance