Have an issue where the heated front seats light turns off after 30 seconds, (no noticeable heat during this brief period) battery is fully charged and have tested with external power connected, to exclude poor battery. Trying the button again, next time the light is only on for a few seconds.

Any ideas?

Unless you have a code reader that can read all modules in the car, it's probably a shorted heating element in the seat. In most cases it's damage from applying your knee on the seat bottom when reaching for something in the back seat.

Front seats, both of them? If its just one i would say the heating element or wiring is damaged. But if both are not functioning I dont know. Have you tried turning them on when youre driving around, not just parked?

It is the same for both seats, and yes, unfortunately the same when I am driving.

If it is both, most likely you have a fault on the wiring or module. Get it scanned.