Does your radio work, but your tachometer doesn't? It is probably your SI board batteries!  

This is a set of two new replacement SI batteries for the BMW E30. They are pre-laser welded to the bus bars for easy installation into your SI board. 

Note: Not all SI boards use these batteries and opt for a different style, so make sure to check your SI board first before ordering. I have attached an example photo of what your SI board should look like to make use of these batteries.