If you have AA Homestart let the AA person sort it out. I've seen one go underneath a dead car with a couple of leads and Bob was his uncle.
Maybe connected it to the starter motor and was able to get enough juice into the system.
Starter is a good bet as it's often accessible. Some cars even have a special terminal for this eventuality.
So long as the live you find isn't switched by the ignition it'll probably give enough power to get the locking going.
Call the AA.
@Hamham started
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If you have AA Homestart let the AA person sort it out. I've seen one go underneath a dead car with a couple of leads and Bob was his uncle.