1. Remove the battery
2. If you have a radio, remove it.
3. If you have a Ammeter reverse the wires connected to the Ammeter.
5. If you have an SU electrical fuel pump it will work with either polarity if it is another brand check and adjust if necessary.
6. Check that both the ground strap and the positive lead for corrosion and general condition.
7. Check that the leads fit the battery terminals correctly , if not replace with new cables (the positive terminal is slightly larger than the negative one), this will give you a better connection, which means more reliable starting.
7.5. Reconnect the battery
8. Disconnect the two leads to the generator. Temporarily connect one end of a length of wire to the positive terminal of the solenoid. Touch the other end of the wire to the F terminal on the generator (the one the smaller lead is connected to, wire is brown with green stripe) several times briefly. This re-polarises the field windings so you get the proper output.
Reconnect the brown/green wire and start the engine. The ig-nition light should act as before: On with the ignition on, off above around 1000 rpm and off when the ignition is off. If the light does not go out above 1000 rpm then try re-polarising the dynamo again. If the ignition light is on when the ignition is off then IMMEDIATELY disconnect the battery and check your work! Reconnect the two leads to the generator, start the engine. The ignition light should act as before: On with the ignition on, off above around 1000 rpm and off when the ignition is off. If the light does not go out above 1000 rpm then try re-polarising the dynamo again. That's it - all the electrical devices on the car will now work like normal. You can install a new negative-ground radio, or other electrical accessories eg a Tacho.By the way, if you have starting problems, check that the earth strap to motor is tight and that all connections are clean. Most electrical problems on Mini's are caused by corrosion and bad connections rather than component failure.