Just before I do this. The alternator connection where the yellow cable is gives me nothing ign on or off. The other gives me 12v live. So before I look to test the cars loom. Should the yellow cable at the alternator give me anything on the multi meter.
The yellow must be the feed to the bulb; on my car it is about +2.6 volts when the ignition is on but the engine not running (bulb lights up, as it has about 10 volts across it); when the engine is running, it's at +battery voltage (14v or so) - the same as the red wire (bulb goes out - there's no voltage across it).
If there's no sign of life on the yellow wire at any time it's possible that the alternator regulator has failed. This isn't a certainty, as I've not checked yellow-wire-voltages without a bulb in - it's possible your troubles are simply the wire isn't getting to the bulb (try another length of wire temporarily to duplicate the yellow wire going to the bulb).
The alternator regulators are available separately to screw into the alternator, but may be hard to find (check which alternator you have, and compare cost with new/refurb whole alternator - millions of the same alternators would have been made, not just for the R4.)
By the way, I am not sure if this is always the case, but at least sometimes if the bulb never lights the battery never charges! The bulb is an essential starting device for the alternator. I had this with an old Volvo - bad connection at the instruments left me wondering why the windscreen wipers were getting slower and slower as I drove along - the bulb had a bad connection, and it had not lit or connected up since I put the key in, so the alternator never switched on and the battery was getting flatter the more I drove on. One hard slap on the top of the dashboard and the bad contact was remade, the alternator started charging and wipers immediately speeded back up (though whacking was only a temporary solution)!