i have 98 GMC Sierra
2 Answers
Make sure battery has a full charge and connections all good. Have you tried a block heater, to see if it helps? Can you keep vehicle in heated garage? Another thought, vehicle cranking, the systems get battery voltage. Engine running, the systems get generator voltage, that can be several volt difference. Check voltage drop for systems. If it cranks good but won't start, check spark--proper fuel pressure and fuel injector pulse. Just my opinion.
Did you install a small battery? If it is really cold you need the most powerful battery that will fit.