Tried this on my X type
I bought some aftermarket seat heater pads from Waeco.com for £GB£45 which came with wiring and switches but wanted it to look like the OE system.The wiring to the switches was there but not to the seats
The main problem was the module which I tried to get made to allow OE timer ie the heat to go off after a while as it does on the X Type
It was over £500 just for the OE switches and the different foam pads (4 needed for the 2 front seats) which have cut outs for the heaters.
It would have been relatively simple to just fit the supplied kit with its own switchs/relay...job done
The XJ is different AFAIK
as it has 3 heat settings and so far my OE ones on my XJ havent switched off unless I remove the key or use the switches.
Cant see why it couldnt be wired using the supplied switches only...while youre on you could do the same with your rear seats....make them heated too...I know someone who fitted heated seats to his X Type front and rear!