1999 Nissan Frontier no start

Lets try below to determine if its fuel or spark problem and comeback with some results

Disconnect a sparkplug wire or 2 and ground it to the engine -have helper crank engine over-do you have a snapping blue spark? If so-you have a fuel related problem, check the fuel pressure to rule out the fuel filter/fuel pump/pressure regulator and listen to the injector/s are they pulsing or hook up a noid light. No snapping blue spark continue to troubleshoot the ignition system-power input to the coil/coil packs, distributor pick-up coil, ignition control module, cam and crank sensors- Note: If it doesn't apply disregard it

In reguards to your O.B.D. Connector, I do not believe it's under the seat. I am currently working on a 99 Frontier, the O.B.D. Connector is under the dash dirrectly under steering wheel. Ck. It out, if you fixed problem, this can still help you for future diognosis. 25 years of turning wrenches, I have never seen an O.B.D. Connector under the seat. Good Luck !