Confusion in transistor circuit
from the voltage measurements it seems likely that unlock is actually a connection to the battery (via a fuse) and lock is a ground connection. making common the input to the central locking controller.







simulate this circuit &ndash; Schematic created using CircuitLab

I have placed R4 and R5 here to prevent smoke when someone presses the opposite control to what the arduino wants. as this is an automotive circuit I have chosen transistors that can withstand 100V or more.

Q2 emitter can be connected to either Arduino ground or to the "lock" terminal.