You should be able to do this without much trouble at all. I have a 4-5/16" mounting depth sub in my 97 cab, which is smaller than the 99's.
I cut a piece of cardboard a little bit larger than what would fit behing the seat.
Then I tried to fit it behind the seat with with it all the way back, I then trimmed a little off, and guess and checked it. Worked out fine.
1997 Ranger XLT GREEN SOLD.
Sony HU: Infinity 6X8's: painted interior vinyl: reupholstered and painted door panels: new vinyl floor: fo-diamond plate behind seats: Suede headliner: Two tone black/tan interior: fiberglass OHC: Black Explorer visors w/ upgraded mirrors: painted grill w/ mesh in grill: locking tonneau: 10" kicker sub: pioneer amp: racing stripes: blacked out tailights: custom paint on rims and hubcaps: Painted bumpers: rocker stripes: More mods that won't fit