The GP light is controlled by the GP relay.
It's located near the battery - see here:
https://citroen.tramontana.co.hu/en/ele ... -part-list - it's #59 in the photos.
Lots of good XM info on that site, as well as suspension, etc in the 'Citroen Technical Guide'.
On the XM, the action of the GPs (pre-heating and post-heating) is dependent on engine (coolant) temperature, so if the car has been run in the previous few hours, the GP-on time may be very short.
When cold, the dash light may go out after a number of seconds, but the heating will continue (post-heating produces a cleaner start than just pre-heating), depending on coolant temp. The going out of the light just tells you it's ok to start.
The GP relay can misbehave if any of its base contacts (or internal ones) are dirty. Best removed, and everything cleaned up.
If you need to replace it, new ones are available (from Citroen and aftermarket) but make sure you get exactly the correct type - there are many variations.
GPs should be taking about 10amp each (i.e. 40A total) if working properly.
If replacing, go for BERU brand.
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Ticking noise in region of fuse-/relay board:
This could be the direction indicator relay chattering.
Metal dust debris inside the direction (lighting) stalk can cause tracking inside the stalk switch, and the relay to chatter.
Try gently lifting and lowering the L-R indicator stalk (without actually throwing it all the way), and if the chattering varies, then the complete stalk switch should be replaced. New ones still available; even the Chinese versions are good.