Where is the bsi unit on a Citroen C3?

For the 2002-2009 (pre-facelift) right-hand drive (UK) version the BSI unit is in the glovebox compartment behind the pop-off panel with a fuse symbol on it. The fuses visible behind this panel are actually on the BSI itself.

The location is not necessarily the same in European models or the post-facelift (2009 onwards) versions.

See the related link for more information. It shows how to remove this unit on the C3 owners' forum.