Blower on my citreon zsara hd estate only works on full blow

Because the first three speeds are not going through the same power feed configuration. You can check the speed switch but I do not think that is it. Kind of hard to fathom three contacts all going bad at same time.

Rather you might want to check the schematic and you might discover that the first three speeds of the blower go through a special circuit powered by your car's ignition switch!!! If that part of the ignition switch goes it affects your ac blower - and don't be surprised if your rear lights and some other stuff is not working also!!!!!

From 06/97; LFX (TU 7 JP(B)); 0.9 Gap
From 03/95; LFZ (XU 7.JP); 0.9 Gap
From 06/95; LFY (XU 7.JP4); 0.9 Gap

In my Xsara it's the ignitionswitch. Seems to be a common problem. Now I need to find a workaround, since the new one won't arrive until friday

Have you tried Googling it on your browser?
Try Citroen's website, or do an image search after typing in the search phrase in your browser's URL search-box.
Maybe something like:
"Citreon fuse box, schematic" or
"Citreon fuse box, layout".
From there you can pick n choose.
Otherwise disconnect car battery, use a continuity tester/buzzer, ring out point-to-point and sketch what goes where.
Eg: wiper switch from which fuse?

Yes it is linked to the ignition switch. Have you checked the fuses for the blower motor?

It sounds like it is time for a new ignition module. Get the model, type and code off the blower housing and go to www.precisionope.com. There you can look up the ignition module under OEM parts.

has your picasso got climate control? if so you probably have a faulty ignition switch
the contacts in the switch which supplies the live to the heater wear and cut the supply

yes its the comms unit, its behind the steering wheel, your car must be a 99 onwards to have this prob, if it is a estate the wires rub on body work, rear doors only, it blows your comms unit, its expensive as needs programming.

check the blower resistor

It will be on the brake pedal assembly ussually at the top where the arm pivots.