Because the car alarm has been sounding off for so long without stopping, it is obviously an old one; current legislation only permits them to sound for thirty seconds before they must reset. Therefore because it's an old one, it is probably a DIY installation also. Most DIY wiring jobs nick the supply volts from the fuse on the indicator lights circuit.
Test my theory: unscrew the indicator lens cap on the front wing of the car, remove the indicator lamp and shove a paper clip or metal hair grip in the socket, this will short the cicuit out and blow the fuse, killing the supply to the alarm.
If this doesn't work, prise open the bonnet and simply disconnect the battery.
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Connect it: Red > Yellow, Yellow > Blue, & Blew to ....'kin bits!
Any ramblings, meanderings, thoughts or musings are mine and mine alone. Any opinions expressed are Lettie's!