Xg falcon with a 4 speed Column. Upon trying to start I get brake fail light and nothing else. In both park and neutral. Have replaced neutral safety switch with second hand one. But still nothing. Problem has gotten worse over the last few months starting from having to held in park. Could this be a smart lock problem is this second hand switch likely buggered.

sounds like it 'could be' adjustment in the shifter linkages may have got worse due to excess wear altho i'm not sure on the set up. other cars i have had was excess play in the linkages which can then mean non starting at various times.

You could test that theory by disconnecting the linkages and then put it in park or neutral or try both (on the box) if it starts no problem it's in the linkage set up itself stopping the box from fully engaging park and neutral.

Does smart lock light go out when key is turned on
Does interior light work
Bridge red/blue wires at inhibitor switch and see if it starts
Bridge power to starter switch terminal (red/blue wire) starter should crank if ok also check power and earth connections
Can you hear the starter relay click when you try to crank. Located under cover near battery has red/blue wire to it

Double post

Bridge the 2 wires at the neutral/start switch that controls it (from memory they are red and blue) if it starts, it's a fault with the neutral/start switch circuit, or wear in the linkages etc. If it DOESN'T start, check things like the connections to the battery, make sure the battery is fully charged, make sure the connections to the starter motor are clean and tight, make sure all the earths are clean and tight, basically just go over all your connections. Also the starter motor relay itself may have issues, an easy way to test that is to swap it for one of the high beam relays (they are the same) and see if it starts then. Also check the starter motor itself.

This video may also be of some assistance https://www.youtube.com/watch?v=BrqkhZmDnHQ

Hope this helps, Gab