Ignition light on No charging

Having finally got my engine back in the Blazer all runs well except the charging system. It's a 1999 4.3 Vortec auto 4x4.
The ignition light comes on and will not go off when the engine is running. I swopped another alternator in and exactly the same result.
I noticed also that the ignition light comes on when the key is turned on to ACC (first click round with the key), none of the other instrument lights come on at this point. When I turn the key the second click the ignition light stays on and all the instruments come to life, lights and gauges etc. Turn the key and start the engine and all the other lights go out but the ignition light stays on and the alternator does not charge the battery.
I disconnected the small single wire from the alternator and when the key is in the ACC position there is no power at the alternator end. When the key is turned to the second position then there is power to the alternator end (as I would expect).
It would appear that there is power going to the ignition light when the key is in the ACC position and that there is an earth as well otherwise the light should not illuminate.
Anyone got any ideas as to where to start to look?

Ok red wire has 12 volts key on or engine running?
Does the big wire to the back of alt have 12 volts all of the time?

Small wire has 12 volts when the ignition switch is turned on or with the engine running. The large wire has 12 volts all the time switch on or off, this is directly connected to the positive terminal of the battery along with the starter cable and another one that I haven't traced.
The main issue that I see is that the battery/charging light on the dashboard comes on when the ignition switch is turned to any position except the off position, so if I want just the accessories such as the radio the light will be on. None of the other lights or gauges come on just the battery light. I checked my Suburban and that does not come on when the switch is in the ACC position.

Small wire has 12 volts when the ignition switch is turned on or with the engine running. The large wire has 12 volts all the time switch on or off, this is directly connected to the positive terminal of the battery along with the starter cable and another one that I haven't traced.
The main issue that I see is that the battery/charging light on the dashboard comes on when the ignition switch is turned to any position except the off position, so if I want just the accessories such as the radio the light will be on. None of the other lights or gauges come on just the battery light. I checked my Suburban and that does not come on when the switch is in the ACC position.
Does the battery voltage go up around 1 volts engine running?
If not.
Have alt checked and or replaced.

My battery light comes on all by itself when the key is in the position between run and off. It always has and there is nothing wrong with my truck. The shift interlock solenoid doesn't work right all the time in my truck so sometimes I have to leave the key in the ignition with it in the middle between run and off (acc I believe) and that little battery light is on. When my truck is running the light goes off which to me indicates the system is good. I have never had any issue from this and my charging/starting system works fine. Get your alternator tested.

Well I have now checked everything and tried two alternators (both from the two vehicles not new ones). The voltage does not increase when the engine is running, infact when I rev the engine the voltage decreases slightly and returns to the original reading when the revs have died down but certainly not charging correctly.
I will get the alternator checked if I can find someone over here to do it, if not I will buy a new alternator at least I will know it is right and if it still doesn't charge then I will have to get an auto electrician involved.

As for the battery light coming on at the ACC position I still find that odd but if that is the way it is then so be it.
Thanks for your input I will let you know how I get on.

If you have battery voltage on both wires at the alternator with the ignition in the ON position: If the light stays on and voltage does not increase when the engine is running, the alternator is faulty.