Pc board

This solution may be too late but will help others with same problem.
The E30 series have 2 rechargeable batteries on the (Service Indicator) SI board WHEN FITTED. located within the dash unit ( tacho/speedo/temp/fuel Unit) they have a life span of 6 years however mine lasted 17years not bad for a rechargeable batt
anyway back to the point the effects of a buggered batts are

• Inconsistent taco and temp guages
  
• background radio noise (old model radios)
• and THE INABILITY TO TURN SERVICE LIGHTS OFF !

The batts can be found in CONRAD (germany) MAPLIN (uk)
the cos of the batts are approx £3
the new board costs direct from BMW with trade discount approx £200 in 2007
have fun (soldering required to replace batts) :-]