If they found the "C" region, you'd think they could get closer to the problem... Do all your dashlights work properly? maybe there's a fault in the switch itself...
I don't know if the speed sensor not working would drain the battery, or how it'd effect the dashlights, but I'd suggest getting a new sensor, or known working one, and inspect the wiring to it.
93 mx3 gs: atx-mtx swap, 94+ spindles
94 mx3 rs: new engine, running again!
88 jeep cherokee: 4.0, 4wd, the 'glitchy ride'.
87 rx7, new and needing lots of love!