You need to steam clean the interior and engine bay.
Post up pics of [all] the damage and [all] the leaks.
Post up a list of [all] issues known to you.
This ain't craigslist. You need to tell us about the skeletons in the closet if you want anything near an accurate guess. Then you can decide to try to sell as is or if you want to do a little work to get more.
What ever some one will pay for it, too me its a run of the mill 3k-5k used foxbody that's had a few hands in it all ready just from what I can see in the few pics, its not an unmolested garage queen you see pull stupid money.
The market right now is really hard to quantify. I watch things on a weekly, if not daily, basis. That said, I see a lot of foxes starting off with lofty asking prices only to greatly reduce them or to leave them indefinitely on the market. It's hard to tell what people are really paying. Yours looks like what you state, and the pics aren't greatly detailed. So, it's a stock-ish high mileage '89 GT, and that you're in Tennessee. Without further data and assuming the best about the car, I'd guess this would sell for $6-7k. That said, the market is kinda crazy. So, if I were listing it, I'd probably ask $8-9k and move downwards from there, if it didn't get people to come take a look.
Agreed with above assessments (with limited information). It looks like a stock GT, missing a taillight with possible e-brake problem, dirty engine bay and interior. I'm with rednotch in the 3-5k range as it sits assuming no rust underneath or hatch/door bottoms. With a GOOD cleaning and detailing (corrosion on battery post, rusty rad support brackets, corrosion on valve covers - to name a couple things) maybe a bit more.
Yep... honestly, @KRUISR and @rednotch are right. I looked at the pics before on my phone and didn't notice the missing tail light, missing head-unit, etc... You should really put a little money into putting SOMETHING in their places, even if it's something off of a parts car or mustang salvage in your area. Then spend a little time cleaning things up. That level of incompleteness will drive buyers away on fears of you not caring for the basics on the car.