E34 have different versions, depending what system is installed. Here some links how to do

BMW E34 Website
kcbmwclub.com • View topic - e34 HVAC Instrument Panel Lighting
Replacing the E34 blower fan and temperature light bulb
IHKA BMW E32 Climate Control Illumination
IHKA LED BMW E32 Climate control LEDs

There are a couple of versions of the control panel. Verify which one is yours.