A little more detail
I'm sure your '88 isn't much different than my '86 so here's how you do it. Remove the two screws on the bottom of the plastic panel, as mentioned. You will then need to remove the upolstery seams from the back panel. To do this without tearing things up start at the bottom of the plastic panel and insert a screwdriver gently into the lower groove and just get the upolstery bead to start peeking out and roll it in an upward fashion to remove it from the right side. Once the tension is released from one side the removal for the opposite side is pretty easy. To put it all back, once again the one side will be easy but the other will require you to pull the vinyl toward you with pretty good force to get the strip and the vinyl back into it's groove. Having someone sitting in the seat and applying backward pressure when you do this will definitely make it a lot easier.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan