my check light wont stay off

Just bought a re conditioned battery for a 96 F-150 XLT SB with a dual tank
gas return system. My ECU and series circuit relay and fuses have all been
replaced. I cannot, for whatever reason get my check light to stay off. Plugs
and wires are new ( I replaced them myself). gas and air filter are not a year
old and maf is fine as well. any recommendations?? I have been fighting my
back pump for about a year and a half now and I am leaning more and more
toward the problem being with either the inertia switch or the computer. And
where is that thing at!!?? please help, thanks