The pre-2001 FL1 could be ordered with seat heating, my own 50th anniversary FL1 had heated leather seats as standard, which was also a 98 model.
They are simple stage heated seats, nothing complicated like 2 stages.
From memory the seats have their own relays (actually under the seats) to control the heater pads, and I think there's a thermostat in the seats too, as I could hear the relay clicking on and off, presumably under the control of the thermostat.
I believe the later wiring will be the same, so a diagram of a 2001 wiring will be suitable.
The seats take a high current supply from the interior fuse box, which powers the heater relays on the seats. The relays are then controlled by the switches on the console by the ashtray, but this is only a trigger supply, so doesn't need to be a heavy cable.
The wiring diagram gives cable sizes for reference.