Twitching gauges and rusty fog light. Advice please.

[jmec]

Hi, Advice please: 1990 325i cabby. 1)The tacho fluctuates a tad across the rev range while the MPG indicator has some violent twitches along with the water temp gauge. Service lights come on with ignition on and go out when the engine starts. The actual revs seem fine and the car drives fine. Am I right in thinking the SI batteries need changing?
2)Front fog light needs the reflector assembly changing. They are ZKW square units. ZKW are mega bucks can a Hella (much better price) insert be used? Thanks in advance for any help.

[Scotty200]

Temp gauge moving is most likely the little brass nut on the back of the gauge cluster come lose or like mine had fell off, sorry I can’t help you with your other queries
Scott

[ah]

Brian moooore is your man for anything E30 electrical.

[jmec]

Thank you Scotty200. The little brass nut on the back of the gauge cluster did it. The one at the rear of the water temp wasn't loose at all but I removed it, gave nut and thread a clean, same with the one back of the fuel gauge and wallah. Tacho, econometer and water temp gauge are working fine.
FYI for anyone with possible SI problems. The SI batteries seemed fine on my car but I researched replacements just in case. They were the Lithium type available on EBay £15 a go; same numbers etc.

[biffer]

Would be surprised if Hella and ZKW lamps wern't interchangeable.