I was having this problem with my 86, swapped out the Back Up Switch in the transmission with one from a junkyard, and lights work. Parts stores wanted $55 for it though, at that price, I'd just wire up a manual one!
Still need this? I was at the junkyard today and picked up the driver's taillight for you, you can have it for $27.58 + shipping.
Good condition, no cracks or chips.
http://charlotte.craigslist.org/cto/3505418962.html
http://i.imgur.com/wuuV2.png
Seems like a good price on an Eliminator, if only I had the cash it'd be mine.