I want to clean the tail light covers on my 92 bird. One side is getting noticeably darker/dirtier than the other. How do I get off the covers to clean them? What do I clean the covers with?

 

Why do you think you need to clean them from the inside? They're pretty well sealed and unless you have big cracks you won't have dirt on the inside.
As far as clening the outside, get clear plastic cleaner. 3M makes a great 2 step system, a cleaner and a polish. Murray's has it and I'm not sure who else. A number of companies make similar products but I found the 3M to be the best. Just follow their instructions on the bottles and you'll see a dramatic difference on any smooth plastic surface.
Hope this helps...

 

where can I get this 3m stuff? Autozone have it?

 

Advance Auto Parts carries it in their Marine/Small Battery section..