Changing from Dynamo to Alternator 

Completing this change successfully, easily and neatly is pretty straightforward on a Mini. The alternator contains the regulator and cut out functions within it’s casing, so makes the regulator box used with a dynamo redundant. However, it is useful to retain the box itself and it’s connectors. It acts as a terminal box to make the installation neat, avoiding the messy and awkward soldering/taping together bunches of wires. It also maintains a period look, plus makes reversal easy should this be needed in the future. 
Disconnect the battery and remove the black regulator box from the car - taking care to make a note of or label each wire removed with it’s original terminal connection. Fit the alternator using the necessary rear support bracket (part no. 12G1053), as the alternator is shorter than the dynamo. The relevant belt (GCB10825) is also needed. The alternator has a smaller diameter pulley to drive it at a higher speed than a dynamo, giving better output and therefore battery charging at lower engine speeds. 
Take the regulator box cover off and dismantle it. Remove all of the coils, contacts and connections to the blade terminals on the bottom of the unit, but retain the blade terminals. Double check that all connections to the terminals are removed including the one to the 'E' terminal, then using some heavy cable - 2.5mm or larger - solder links between terminals A, A1, and D, connecting all three together. Now solder a link between terminal F and E. Replace the box in the car and re-connect all the wires to their relevant terminals except the ignition warning light wire (brown/yellow) which must now be connected to terminal E. Connect the ex-dynamo wires to the alternator - large connector to the large terminal (output connection) and the small one to the small terminal next to it (sensing terminal). 
If the car being converted was already negative earth, re-connect the battery and away you go! If the car was positive earth, you will need to reverse the connections to the battery and swap the connections on the ignition coil. 
If there is a radio or cassette player fitted, check to see if these can be re-connected as negative earth. If not, then unfortunately they will need replacing. Everything else should work as normal.