One other thing not to overlook on vehicle like this is possibility of an aftermarrket alarm that uses one of its immobiliser circuits to cut the S wire to the solenoid, in that scenario that needs be inspected/tested if S wire power is found be the suspected issue . Even the UK/EU approval requirement alarm upgrade via Sigma had starter cut via S wire . Relays on the Sigma control boards known be problematic at 20 years old .

If you find it works with battery charger connected I would assume it poor connection and voltage drop issue. Could be just coincidence & it always best assume nothing and test everything so the S wire to the starter solenoid wants be proven be always good when key turned to crank . A quick way determine ground side issue is use a jump lead from battery to block ground and test starter function, test the battery terminals for voltage drop first though . You only got 2 + powers to starter, main heavy direct from battery (no relay as the solenoid is the relay) and the solenoid control wire fed from ignition start position . Use a test light and/or DMM to test for poor connection at various connection points, test battery terminals too, don't forget to test both sides of connection terminals/crimp points etc to try determine exact point of voltage drop or perhaps no solemoid control voltage from ignition when turned to crank . Don't forget voltage drop needs be done on a loaded circuit, if you don't really understand voltage drop testing youtube will help teach the basics . A link to a basic starter test principle, same test basics can be used on any electrical connection/circuit with some cognitive thought to find problems . https://www.youtube.com/watch?v=_xvddRJmkAA Once you understand the principles the testing becomes easy & you don't throw new parts at it as you can do diagnosis rather than tryagnosis ...

That is a very odd situation with the battery charger affecting the starting issue. I do wonder if it is placing a supplementary earth to the battery in which case you have an earth issue on the power supply side of the electrical system?

Hello, i hoping someone can help with this. (I'm a Audi master tech, but Subaru do work very differently as I've learnt over the years of owning this car) So recently my car has been having a non cranking issue, I'll turn the key but the engine will not turn over. sometimes i can get it to work if i turn the key a million times. I am getting 12v to the starter motor switch and can hear the starter make a click. beck of the findings i decided to replace the starter motor, and i also did a new battery at the same time. unfortunately I'm still getting exactly the same issue. when the fault is happening if i short the starter motor out it will start the car no problem. this has made me think there's an issue with the relay, now I've not been able to find anywhere that tells me the exact relay. i have pulled all the relays out from under the drivers dash and have still got voltage going the the starter, so not sure what to do with the relay. I have also been getting a stalling fault at the same time, when i push the clutch the car will stall. i found that the clutch switch is on the same system as the starter system. I replaced the clutch switch with a new one from Subaru, and this has help a lot, am no longer getting the stalling issue and the starting issue has been reduced. back to the starting issue. so when i fitted the clutch switch the car still wouldn't start unless i used the power probe in the starter, but as soon as i put the battery charger on it started 1st time. i have checked the battery which is testing perfect, and the alternator is charging as expected. I have been able to reduce the fault from happening but fitting my motorbike solar panel trickle charger but does pop up now and again so the underlying issue is still there. I am running out of ideas of what to check next and would greatly appreciate any advice and help. Plus would really like to know what relay and where they are in the car (believe there are 2, B422 and B225 but not sure) Thanks for any help. (my car is a 2006 Subaru WRX STI PPP)