Depends on the brand of the radio.
AFAIK, Philips and Clarion were used  by Citroen in the Xantia.
For Blaupunkt radios there are adaptors.

What kind of radio did you get?

Claudius

greatmusos@btinternet.com haute in die Tasten:

The Xantia factory Radios have a front panel which is wider than the
usual DIN slot. So you will need an adapter to fit your radio in -->
eBay. Wiring should be quite easy.

The steering wheel controls is a different job. The Xantia steering
wheel works with resistors, which create different voltages. The
radio interface meters the voltage and detects which button has been
pressed. This is quite a simple technology which has been used by
some manufacturers. Unfortunately there is no standard setup for the
resistors. If there is a remote control for your radio which happen
to work after the resistor principle, you could try to find out the
correct switching voltages, open the steering wheel and install the
appropriate resistors for each function - which sounds to be quite a
lot of work to do.

If your radio happens to be a Blaupunkt, chances are good that there
is an aftermarket interface adapter available.

Frank