The clicking sound is the solenoid on the starter. Either your battery is very dead, or the positive cable going to the starter is in very bad shape, or the solenoid/ or starter is no good. not much to go wrong here.
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Baffles in the fuel tank are fitted in order to?
Could be heater plugs if its a diesel or maybe the battery. Check the earth (ground) cables between the engine and body for cleanliness and security of connection. Turn the ignition key, you should hear the starter solenoid 'click'. If it does, the cable from the battery to the starter is a fused link, connect a starter cable between the battery and starter motor solenoid and try to start. If there is no 'click' the fault is either the ignition switch or the wiring from the solenoid to it. If the car is automatic it should only start in Park (P) and Neutral (N). check the operation of the gearbox start inhibit switch. If an aftermarket alarm is fitted check the security and cleanliness of the wiring connections, especially the immobilizer circuit.
Most starter motors are located in the bell housing. They mesh with the ring gear fitted to the engines flywheel. Starter motors are cylindrical in shape and are about 9" in length
Defective starter (brand new broke) Is in gear (try in neutral) Seized engine (can you turn it by hand? Does a new engine help?) Seized accessory (try removing belts) Hydraulic lock (fuid in cylinder, try removing spark plugs)
Possibly the ignition switch is defective.
Starter button ? - you must have a very strange heater, they normally heat automatically when power is on, and I have fitted hundreds and NEVER seen a 'starter button'
Try and turn the Impreza engine over by hand. The engine could be locked up and not let the starter turn it over.
The fitted CD player has a CAN-Bus interface built in for remote control and speed volume adjust. If a third party CD/Radio is fitted, these functions are lost, along with the trip computer. It will not damage the trip computer.
Your mechanic has the starters in his hand. How can a website know more than he?
Disconnect the battery, negative first and then positive. Then disconnect the wires on the starter. Supporting the weight of the starter, undo the bolts that hold it and withdraw it from the housing. Put in the new starter and fit the bolts loosely. Tighten the bolts carefully, ensuring the starter is fitted properly into the housing. Reconnect the wires making sure they do not lie against any metalwork or get trapped where they might rub through. Finally reconnect the battery; positive first and then negative.