I have a 1998 C70 schematic, so are assuming MY 2000 is identical.
Look under the seats for the wire that feeds the heater elements. For the left seat, it is yellow with a red stripe. for the right (passenger in the USA) seat, it is green with a brown stripe.

You need a relay for each seat. They are the Bosch automotive type you can buy at the Radio Shack for $6 each, and plug in behind the instrument panel, second row of relays from the top, 2 leftmost positions.

If you remove the blank switch covers, you can check if the connectors are secured behind them. The wire colors are:
Left switch . . . Yellow/red, violet, blue/yellow, black
Right switch . . . green/brown, violet, blue/yellow, black

Note: blue/yellow = blue wire with a yellow stripe.