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On the manual trans it should be a switch screwed into the case, or with your trans probably the top cover.
I can't remember the last time I saw an NP435 trans up close and personal though, so might be off. I'm sure someone here will have a detailed shot of one for you.
I mentioned NP435, and in '78 I think that would have been the only manual. But they did offer two different granny low manuals at one point at least, so we may have to verify which one you have if it turns out you could have had either one in '78.
If it comes down to it, do you have the floor tunnel cover off for access? Or can you get a good shot of the trans from underneath?
For some years the shift pattern on the **** will tell the tale, but in '78 the NP had the later style shift pattern I believe, so not a guarantee.
The T18 & NP435both granny transmissions have the BU switch in the top cover.
T18 red switch in the top cover to the rear
NP435 again to the rear of the top cover. That is the Advance Adaptor overdrive in front of the NP435.
On my 81 F100 the harness run along the transmission cross member to the frame rail.
If you short the 2 wires together the lights should come on.
Dave ----
backup light switch
