The GMT-800 auxiliary heat fittings are a 10-year/100,000 mile expected lifespan part/s as is the 3800 elbows, GM should/could have done better.....But they didn't.
On GMT-800 pick-ups.....The plastic adapters only break when manhandled or bumped into, My 400,000 mile 2500HD still has the originals on it; One will blow off now that I bragged about it!!!
GMT-800 with gas engines......Knock sensors, Intake gaskets, EVAP vent valves, Compressors, Broken wires at the throttle body harness, Broken exhaust manifold bolts.
GMT-800 with Duramax/Allison.....Trans cooler lines, Range Sensor, LB7 Injectors, LLY Injector control wires rubbing through.
GMT-800 (General) HVAC actuators, Intermediate steering shaft, Transfer Case Pump Rub, Wheel Bearings.
Interesting fact......I have NEVER replaced a main heater core or a evaporator core on the thousands of GMT-800 platform trucks I've worked on!
Dodge 3rd Gen, The vast majority are diesel......Injectors, Heater Cores, Evaporator Cores, Broken HVAC doors, Compressors, Radiators, Fan Clutch, Wack-a-mole on suspension parts, Lift Pump & relocate kits, Governor pressure solenoid & Transducer on 47RE/48RE.
Ford Truck.
Gas Truck.....COILS & plugs, 5.4L 3V VVT Phasers/Timing chains/Tensioners/Guides, Injectors, EVAP leaks from the "capless" filler necks, O2 sensors, Vacuum leaks from the "natural rubber" PCV-Brake booster components, Catastrophic compressor failure since they went to TXV.
Diesel....
6.0L....Injectors & Hold-downs, Oil Coolers, Early HPOP's, Head Gaskets, EGR Coolers, Degas bottles, FICM(low voltage), Sticking Unison Ring on the VGT, FUBARed injector harnesses from prior idiot's, Engine Harness repair.
6.4L....Radiators & Cab Mounts, Injectors, HPFP, Rocker Arms, Engine, Turbos.
6.7L....Complete fuel system overhaul from CP4 injection pump failures, Turbo.
I would include the common transmission failures.....But the list would be LONG!