Where are the glow plugs located on a citroen relay?

The Jumper/Relay began manufacture in 1994 - by that time, highway diesel motors had almost universally gone to either common rail or direct injection, which eliminated the need for glow plugs, so there's a good chance that there aren't any. We would need a specific model year to know for certain, as several changes have been made to the design over the years, and the specs for the 1994 model will differ greatly from a vehicle which is a 2011 model.