Brian is correct. I started a thread last year regarding the Aero Tanks 60 gal. replacement fuel tank and someone posted that he had a miserable time installing their underseat storage compartment.
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Another alternative is the lockable Mopar underseat storage compartment sold through your Dodge dealer which is different than the optional underseat storage available as a factory option since March of '98. The Mopar units are color matched to your interior and available for both Quad and Club Cabs. Retail prices on these are around $200. for QCs, $220. for CCs. I posted a thread on this over the Fourth of July weekend last year.
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'98. 5 2500 QC 4x4, (BLACK) SLT Sport, ISB, LWB, 3. 54 LSD, A/T, Glasstite Vision II canopy, Line-X bed liner, 285/75R16 BFG A/Ts on 16x8 M/T Challengers, Warn 4X Boards, Mopar fender flares, front and rear NW Custom stainless steel/rubber mud flaps, Mag-Hytec diff. cover and trans. pan, Edelbrock IAS shocks, Mopar tow hooks, Jordan Research Ultima 2020 trailer brake controller, VDO Vision gauges, PIAA Dual Sport 900 auxiliary lights and Super White bulbs, BD exhaust brake and TorqLoc, Prime-Loc fuel filter relocation kit, Bosch 275 hp. injectors, VA CPC, DD Boost Module, Banks Stinger wastegate actuator, "Scotty Air System", BLACK '00 valve cover, Hitco/Geno's exhaust blanket, Femco/Geno's EZ Drain oil pan plug, Banks Stinger 4" dia. stainless steel exhaust system, plus many other trick modifications
Coming Attractions: BD "Medium Stall" TC, BD valve body, BD PressureLoc, BD trick modified front pump assembly, '00 Sport headlights, '00 Sport grille (I'll squeeze it in somehow)
'00 Polaris 6x6