Id bet its in the door latch itself like in the door jam. Id spray it with some wd-40 and work it a few times. Theres really isnt anything else internally that could mess up.
2006 Silverado 2500HD LBZ CC SB
Exterior: Escalade Handles, Front windows tinted to match the rear, painted bumpers & mirrors, billet grille, 305/50/20 Cooper Zeons on BMF Novakanes Death Metal, Cognito Braces, Stainless Sleeves, Train Horns
Interior: 4 Kicker 10in Subs in 2 custom box's, Autometer Ultra lite II Pyro and Boost gauges, Aeroforce Interceptor Gauge, LED Lows & Fogs, Painted accents
Performance: EFI Live Tuned by Blackout Performance, Custom Blackout Performance S475 over stock twins, 45 over stock injectors, MBRP Turbo Back Stainless Exhaust (Straight piped), Raptor Lift Pump, AFE Filter, GMAX6 kit with Suncoast 1057 converter installed by Blackout Performance
1999 Ford F-550 PowerStroke ''Wrecker'' **Jerrdan bed, S&B Intake