Try tracing the wires back to the control relay and cleaning up all of the contacts on it. When I had my diesel 205 the relay sat low enough down to get salt etc off the road blasted on to it, and the ensuing high resistance contacts prevented the glow plugs from operating. A good clean up fixed it. At least it's free to try.
I would have thought that the relay is the same for most of the Pug/Citroen diesels. Certainly mine was marked with Pug, Citroen and Talbot logos on the part...