Temp sender wire is purple/violet. You could ground it in the engine bay and turn power on. Gauge needle should travel to 'H' if limiter is working.
Brake warning lamp and oil warning lamp get their ground at those remote switches so instrument housing ground would not be a factor for those. The high beam indicator lamp does ground through inst' housing, as do the illumination bulbs and turn indicators, so the inst' housing is apparently grounded. There may be another bulb on the illumination circuit, radio dial? , shift indicator? Any of them work? Should be one fuse in the fuse box that doesn't show power on either end of it until the headlight switch is pulled to at least park lamp position. If the fuse and dimmer rheostat are good your test lamp will brighten and dim as you turn the switch knob. 90% of the time that rheostat in the headlight switch is the fault.
