If you don't have one already, access the electrical section of the factory service manual for you vehicle at NICO Club's site, which you'll find here:
https://nicoclub.com/FSM/Frontier/1999-VG/el.pdf
Fusbile links are in the Fuse & Fusible Link Box, which is pictured on page EL-15.
The ignition switch gets its power from the 40A fuseible link "e" on the green wire (see page EL-13). The ignition switch's Red/yellow, black/yellow and green/yellow wires are all outputs of the ignition switch (see page EL-14). So, if you have no power to the ignition switch at the green wire, you have to check 40A fusble line "e." If the fusible link is "open," then replace it. If there is no power to it, then check the circuit between it (in the fusible link box) and the battery. If you have power through the link, but not at the ignition switch, you need to check the circuit between the fusible link (in the fusible link box) to the ignition switch.