My XJR instruments have gone kooky on me lately
I will keep it brief
3 weeks ago I had a new Alpine hands-free stereo fitted, with amp, and 4 x 5 1/4" door speakers, great system, but not enough bass, so subwoofer to follow, but I digress..

I got the car back, no instruments in the binnacle worked (revs, speedo, temp, oil, fuel, Volts). the stereo installer couldn't fix it so took it to my mechanic, who removed the instrument cluster and found the big plug loose. problem sorted for 2 weeks

past few days, AIRBAG warning comes on, then off after a while.
Then today no instruments at all.
Then they came back later in the day.
Then, driving 60-70 km/h, they went AWOL, and volts and fuel off the gauge, temp/oil on zero, and revs stuck on 4000, speedo on 140km/h. This stayed liked this even when I turned the car off( and I never got anywhere near 140 on that trip)
Later it all came right.
I suspect the plug is loose again, and will need the cluster off again. This time I will supply a zip-tie to hold clip together! this is costing some $$

the weird bit - I wonder if the mid- 90s X300 has speed-sensitive power steering. I doubt it.
But when the instruments were frozen on 140km/h, the steering was really heavy, but still powered. Once the instruments came right, the steering was back to normal

 

The manual and adverts claimed it was speed-sensitive. I guess your head unit would've turned the volume up, too were it original. Not certain, but your wipers, if on intermittent, then, wouldn't have paused very long; think they're also speed sensitive

 

Yep- there is a cable to the rack. You have good senses!

 

AL NZ,

The ZF/Bosch Servotronic Variable Power Steering incorporates a control module behind the LH A-pillar trim, and a "converter" or "transducer" mounted on the steering rack pinion housing. But since the control module relies on a vehicle speed signal from the Instrument Cluster (IC), the steering issues are almost certainly related to the rest of your IC issues.

In addition to the large connectors on the back of the IC, it would be worth checking the LH Console Ground Stud, which I think is on the left side of the transmission tunnel or on the bulkhead/firewall. That is the point at which the IC is grounded. It's possible the stereo installer disturbed the ground stud or the wiring attached to it.

Cheers,

Don