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Instrument panel light wires are all colored blue with a red trace. The socket that lights the speedometer should be found just north of where the speedometer cable screws in, between the fuel and water temp gauges (assuming you have the round gauge cluster and not the Custom Cab one). The socket just above that one is for the high beam indicator and the wire for that is green with a black trace. Hope this helps!
The comment regarding all instrument panel light wires being color coded blue with a red stripe still applies. It's been a few years since I took my Custom Cab cluster apart, but from memory I think the speedo has two light sockets, the temp and fuel gauges have one each, and if you have the optional oil pressure and charge indicator gauges they also each have one light socket. So look for the blue wires with red stripes terminating in light bulb sockets.
As mentioned, Blu with red tracer should be correct wire color code for the dash lights. Looking at sweeping speedo; there are the two sockets at the bottom for the oil and alt idiot lights. Offset and slightly above idiot lights are the 2 instrument light sockets. The upper top 2 sockets are for the turn signal light, and the other is for the low/high beam.
Mentioned 'fuse'; possibly the issue is with a faulty headlight switch: Instrument lights operate off a rheostat, turn the **** to full counter clock position the instrument lights are off, gradually turning the **** clockwise increases the brightness of the bulbs. If the issue, suggest clean rheostat or replace switch? According to 1965 shop manual instrument bulb is an 1895.
Most sixties cars (all I've owned, and that's quite a few) have a red indicator for high beam.
Not sure when blue started for high beam, yet I think it was well into the 80's, and probably with the japanese. My '83 Honda GW has a blue indicator, yet my seventies Honda's were all red.
On my 65 CC there is a round red high beam, and green for right and left turn signal plastic film insert that sets on the back of the panel which is usually missing so one may find red film is used for both high beam and turn signal or Green/Green, imagine what ever flavor one prefers I acquired 2-3 spares and none have the same configuration?
If U do not already have the 2 mission instrument light sockets with the leads U should be able to purchase thru a Ford part supplier, to include a/the Constant Voltage Regulator(CVU)
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light bulb in the 66 F100? I can pull the dash and easily get to the OIL and ALT lights but can't find the dash bulb. 
66 f100 dash light
