Hi kesty and welcome to the forum
As has already been mentioned, the connections are behind the rear left side panel, but be aware that not all will have the connections fitted from the factory.
Mine is a 2010 110hp model and mine already had them.
Take the panel off and look for these three connections below.
To connect to the Citroen wiring loom, your towbar wiring would have come with a fusebox that looks like this
And the towbar wiring will have plugs on the end that look like this
The three connectors above are for a 13pin or 7+7pin (12N/12S), so for a single 7pin (12N) you won't have connector no.4
If you don't have the last two/three connectors and the fusebox, then you don't have the genuine Citroen towbar wiring and probably won't be able to connect your wiring to the existing connectors without some jiggery pokery