Where do I start to check lack of power to anything in my truck?

[reliefaswas]

`93 Ford, F-250, diesel, batteries both up (12.5 volts) test light lights when point on pos. (+) and clip on block (-)...fuses inside cab are all OK, and large fuses under hood are OK (power distribution box) The key does nothing and the headlights won`t go on. PS: ignition is bypassed to 2 manual switches due to ignition key failure. (still starts by turning key to engage starter though) Fuseable links? (if so where are they located)