Which side to fill up car

Did you replace the fuel level sender in the gas tank; the one with the float on an arm that swivels and causes a little wiper to go across a resistance material? I've seen the little wiper break off and I've seen the resistance material worn through.

The one on the left has the ejector attached so that fuel will transfer from one side of the tank to the other. Are there any codes stored for the DIM?

Nine times out of ten, it is the sending unit in the tank that is the problem. To check, use a sort jumper wire to ground the gas gauge wire to the frame while watching the gauge needle on the dash, with the key in on position.

Find the gas tank electrical connector by the tank. Pull it apart, and on the harness side of the connector, locate the wire going to the sending unit. I don't know what color it is, but the fuel pump wire is a pink wire with black stripe and there is a black wire for the ground. The other wire should be for the dash gauge's sending unit. Have a helper turn the key to on and watch the gas gauge for any perceptible movement while you use the jumper wire, insert one end into the terminal for the gauge's sending unit wire (this is on the harness side of the connector, not the gas tank side), and touch the other end to the car frame-you are grounding it. If the dash gauge is good, the needle should move a lot, usually will move more than half way, one way or the other. So if it does, you know the dash gauge is good, and the problem is in the sending unit.

If it is the sending unit, it is quite a job to drop the tank, pull it out from under the car, then pull the fuel pump and sending unit out. You might be fighting a lot of rust on an '89-get some penetrating oil. And look for the removal procedure in a repair manual. It's not terribly difficult if you are handy with tools...and like to get dirty. Those hoses on the tank have been stuck on there a long time. Good luck.

If the trouble with the gauge started after you worked on it then you likely damaged or created a restriction to the movement of the sending unit. You will have to remove the tank and double-check your work.

The fuel pump relay is located below the right side of the dash to the left of the engine control module. There is also a 15amp fuse
for the fuel pump. In the fuse block under the left side of the dash
fuse #19. Hope this helps.