You will need 2 AA nicad batteries and a holder and a little bravery. You will be opening up the dash and removing the current batteries paying attention to the polarity and soldering the leads from the battery pack into the board. Total cost is a little over $10.00. There is probably a lot on this in the archives and also a really good write up on this, step by step with pictures on the unofficial BMW E30 homepage (I forget the URL but if you do a search it will come right up).