Had exactly the same problem on our '98 Avant a couple of years ago. On ours both rear calipers were seized and had to be replaced (Eurocarparts). Lack of use was blamed. I think adjustment is automatic on these cars.

If you're very lucky it might be just the pads are seized . I'm sure someone else will be along to offer help on this. I seem to recall that some special caliper piston wind back tool is needed. As stated above on our car it was a case of replace the calipers. I had the rear rotors and bearings done at the same time.

Kind Regards.

Paul.