Disconnect or remove your aftermarket alarm system.
Is there any chance your brother installed the battery with +/- cross connected even for just an instant...??
The other thing that might be is that one of the MAIN electrical system CBs (Circuit breaker) might be opening due to a short somewhere downstream. This self-reseting CB will cycle on and off rapidly with a downstream short. I would recommend pulling ALL of the fuses, being careful to note which goes back where, and then reinsert them one by one until the CB begins cycling.