display darker and fickers after disassembly

hey

i changed the HDD in my imac to a SSD for that you have to unplug your display to get to the HDD. Everything worked fine and my imac is running perfectly. The only problem is, that my display now isn´t as bright as before and sometimes it gets brighter for a few moments and then get darker again, but its doesn´t happen very often. Is the displaycable maybe not connected propperly? I haven´t opened it yet because the tape to put it back together is a bit expensive.

Thanks for your help.

Johannes