While I know it will be a PITA, can you put the old box back in to confirm it works fine with the old one and bad with the new? That would also rule out asssembling it wrong causing the issue. Just my .02

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)