I have a g5 but your flasher unit is probably in your left toolbox. With the box open and the trafficator blinking you might be able to hear or feel the relay click to know it's the right little black box.
I know they are different but the G5 locates the turn signal relay in the left hand triangular side box. There are three little black plastic boxes in there and the signal relay is the only one with three wires connected to it. The terminals are two in line and one at 90 degrees to the other two.This unit is a solid state device that does not make a clicking sound when it is in operation.Again, knowing it doesn't apply to the C5, in the left hand hot dog container are three larger plastic relays. I mention this because the smaller size of the turn signal unit is one of the things to look for. That and the three wire connection.
It's not difficult.the single kit you ordered is all you need. The webpage shows 2 diodes already connected together at their cathodes.
Hmmm..... the black wire (ground) should also have the black with green stripe from the indicator lamp (speedo side). The other side of the black with green stripe from the connector should be connected to the diode.
one more diagram ....... this is the wiring modification.
Good Luck, let us know how things turn out for you.