The 3D slider is attached to the speaker ribbon and connected to the main PCB in the upper right-hand corner. If you didn't disconnect any of those three ribbons (speaker/3D slider, camera ribbon & LCD ribbon) then that's not exactly your problem.
If you did disconnect any of those three ribbons (top right corner of PCB when you've removed the back shell) check there to see if the flex ribbon is not completely flush with the port (See image link below of location)
3DS Slider & Speaker Ribbon Location
Now, it sounds a little vague but you're saying that the slider moves completely up & down with no change to the 3D aspect? If this is the case, the 3D slider may have been damaged at some point. Did the slider ever get stuck or not move and was forced back into a "working" sliding order?
I don't believe there are any options in the settings area but you should double check there before doing heavier repair work to the 3DS. If there is some random option that lets you completely disable the 3D effects, check there first. If thats not the case, then I would suggest the next part.
Either one of two things has happened:
- Your plastic slider has broken and is not grabbing the black bar that adjusts the 3D effect
- The internal 3D slider has broken
From that, you'll need to tear the system down and get into the top portion of the 3DS. It's a bit more advanced but possible. You'll want to follow this guide in order to get at the parts you need to inspect first: Nintendo 3DS Speaker Assembly Replacement.
I'm going to guess that the 3D slider component is shot though. Luckily it's a pretty cheap item off eBay. If you decide to go that far to get the 3D working, just take your time, work slowly and be patient. Nobody wants to do more damage than is already there.