A quick test will tell you whether the alternator is sending charge to the battery. If it isn’t, then you need to start checking the wiring and things like the whether the car is earthed properly. Perhaps there’s a blown fuse of circuit breaker that is preventing the flow of charge from the alternator to the battery.

If the alternator is, in fact, charging the battery when the engine is running, then you’re looking for a problem such as an aftermarket alarm or a boot light or some other electrical gremlin that is draining the battery overnight. It would also be worth checking that the new battery and alternator for correct operation and capacity. It wouldn’t be the first time a brand-new component has been faulty, fitted to a car and then confounded the mechanic responsible.

Change the belt as recommended at six years. Belts deteriorate with age as well as kilometres and you don't want to risk the damage that might occur to the engine should the belt fail.

It sounds plausible that the wiring harness could be damaged and needing replacement. You can get your answers from Carsguide.com.au.