Grouch,
I had the same problem when I bought my truck. I was able to get it diagnosed and fixed under a used car warranty provided by the dealer. In my case, the switch on the steering wheel was good, but the electronics on the servo were bad. I'm glad that the dealer had the ability to track it down. If your switch is good, you may want to have the tech diagnose the problem part, as there are several electronic parts that could be causing the difficulty.
Sorry that I couldn't be more help,
Steffan Marley
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'93 W250 CC, 4X4, 3. 54 Gearing, NV4500 5sp,
16cm2 Turbo Housing, Pacbrake, Amsoil Bumper
To Bumper W/Bypass, Boost/Pyro/Oil Temp Gauges,
3. 5" Exhaust, HP Injectors, K&N Filter, Hella
Lights, Icom 2 Meter and Uniden CB Radios.