240D headlight switch accidental modification

W123 main light switch does not control reverse lamps. If they are not working, check fuse 14 (it also powers the horn, so test your horn before removing fuse box cover). A failing neutral safety switch is common cause of inoperable reverse lamps. Instrument cluster (gauges, speedometer and clock) are likely not working because the dimmer is corroded or damaged. Try twisting the dimmer knob back and forth (it is next to Brake warning lamp. The knob is also the button one presses to reset the trip odometer). Dimmer does not affect the gear-shift and air-conditioner lamps (the latter is next to blue temperature knob). If either or both of these are lit, but cluster is dark, this likely indicates a failed dimmer control. Some people bypass the dimmer, others attempt to repair it while brave folks pay the bucks for replacement part. Its up to you as to which method suits the need. FYI: license plate lamps are also fed by power that illuminates the gear-shift and air-conditioner lamps, so looking at license plate lighting will help validate main light switch wiring.