Hi Marc,
Its an easy mistake.
I found this info regarding the problem that happened on another XJS
your alternator diodes will be fried
electronic ignition module is likely to be fried
if it has a fuel injection computer, that will be a concern
other items such as sensors, motors etc “should” be ok
The EFI relay has a diode in it to prevent the relay from energizing if the battery is hooked up backwards. If you have the correct relay in there, it should have protected your ECU.
Hope its OK and the ECU was protected.
Regards,
Julie.