Rear Clean in Renault 19 TD

The rear windshield wiper only works the first time I turn it on, and only once. After that, it stops working. I've cleaned the switch with compressed air in case it had carbon buildup, and the connections are fine. What could it be? A relay? Where are the relays located, and which one is it?

The relays are located in the same fuse box or at the front near the motor. This relay is usually in the fuse holder. It could also be that the motor's contacts are worn. Remove the top cover and check if the points or contact channels are worn. Alternatively, you could try another motor and test it. You can also check for continuous current flow at the connector.

The relays are in the fuse box, but which one is for the rear wiper?

You should have a sticker that tells you which one it is, or activate the wiper and listen to where the relay activation noise is coming from, or test if there is power at the back, at the motor connector.

Bridge the wiper relay, remove it, and connect pins 30 and 87 with a wire. If it works continuously without any issues, the problem isn't in the motor or any wire from the relay to the wiper. Start there for now.