I never really tried using the shifter on my DFP, since I'm used to the flappy pedals. Now I tried using it, but both shift "buttons" do not work at all.
I have the wheel here in front of me without the upper plastic cover, and every cable seems to be connected properly. I used a pen to press the buttons under the shifter stick, won't work too.

Is there maybe a "hidden" key combination to activate them?

No they should be working by default. You can't assign them to controls in LFS?

Well I just unscrewed the small platine that holds the downshift button and its almost broken in two... the circuits are heavy damaged, needs a bit of soldering. The first owner wrecked it and than sold it to me on Ebay, well thats live

edit: tried to make pic

soldered it, works now

I've never seen damage like that - what the hell did he do to it?

I guess he downshifted like mad...or let the wheel fell on the stick, either way he sold it to me without telling, I never used that stick so it can't be my mistake (I neither let it fall) But now everything is working again