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Your suspicion that the starter relay could be the cause is already quite correct. This is where I would start first.
The relay is relay number 3 on the relay box in the engine compartment. Check if the relay (if the engine does not want to start) is supplied with voltage, i.e. if there is current on the control circuit.
If not, it can also be the ignition lock.
If yes, look if the relay switches, if not you have to change it.
If the relay switches through, it is not defective and it must be checked whether Plus arrives on one side and is passed through to the other. If so, check whether the starter motor is supplied with voltage. If not, check the cable.
If so, the starter motor is the cause and must be replaced accordingly.
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