changing lightbulb

Yesterday the boardcomputer of my Audi A4 (limousine, 2003) indicated that one of my rear lights gave up.
I tried to change the light bulb, only to find out that the manual tells me
to go to the dealer to have it changed. I refuse to do that and I tried to do it myself but that is not easy! The cables and bulbs can't be reached from the trunk anymore.
I found a screw in the trunk close to the left rear light unit. When I remove the screw, only one side of the light unit is free and can be moved. The other side is still fixed, but there is nothing that I see to losen that side.
Does anyone have a suggestion or experience?
Thanks, Dirk
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