Stephan,
Assuming that your alternator is indeed a Lucas 11AC (i.e. Stock), the 3AW unit should be connected to the AL terminal of the alternator. If you look at the back of the alternator, the AL terminal is the one on it's own located furthest from the engine block. The wire will go into a loom of 4 wires from the alternator which is strapped to engine frame and leads backwards towards the battery area. Could you check if the AL terminal on the alternator is connected in the way described?
The puzzling thing about your case is that though the 3AW is disconnected, your ignition lamp shows some signs of life - i.e. it is out most of the time, but comes on occasionally. This clearly implies that it is connected to something, but not the 3AW. So, here are a few tests you could do to help diagnose the problem:
1. Does the ignition warning light come on when you turn on the ignition but do not start the car?
2. Does the ignition warning light go out once the car is started?
3. As above, is the AL terminal on the alternator connected to somewhere?
4. Look around in the area under the heater box or behind the regulator. Is there another device hidden there with 3 wires connected to it? I'm asking, as many of us have had 3AWs fail, and have replaced them with a solid state device available from Coolcat Express. This device can be hidden away in several possible locations and replaces the 3AW functionality.
Maybe you have a solid state 3AW hidden away somewhere, and your problem is (as George suggested a while ago), that the cable from the AL terminal on the alternator is flaky, and making intermittent contact.
Best of luck tracking this down.
-David