you mentioned that the pump starts as soon as you turn on a tap.
this implies that it wasn't running though it had power before you turned on the tap.
if that is the case then the pump must be building pressure and tripping out
the pressure switch unless it is also defective. If that is the case then you
likely have a blockage in the output line between the tap and the pump or
in the accumulator tank (if you have one) or in the small orifice in the pump where
most incorporate a backflow valve and pressure regulator
could still be in the input lines but it it is building pressure and shutting down that
would be my second guess.
Need some more info (what kind of pump, does the pump shut off again when the tap is closed? Try the aforementioned test of running the pump with noting on the output side.
i've been living aboard
for over a decade and even my home ashore is off the grid
all 12V pumps for water supply and transfer. I find them to be very reliable
if they are kept cool (don't put them in a tiny box and run them into thermal shutdown) and if they are used. Letting them sit as yours has can do everything from
simply clog them to sucking the valves around backwards or causing them to leak and
in the worst case strip out the gears on the wobble plate in which case its pretty much trash.
luckily, good pumps are now phenomenal cheap
compared to what they used to be.
I paid $400 for my first 4gpm water pump. Now I can get one that is more reliable,
smaller, and draws less power for about $150.