If there's more than 1cm of fore-aft play in the shift lever then your first step is to fit new shift rod bushings. I expect you'll look under the car and say, "What bushings?" There used to be a plastic bushing at either end of the rod that connects the bottom end of the shift lever and the transmission.

If shift linkage is tight, and in my SDL is there is practically no play with new bushings, the next thing is to adjust the neutral start switch on the side of the transmission. You can't miss it. It's the plastic box held by 2 10mm head bolts. My adjustment rule of thumb is to set the shift lever in reverse and have someone yell out whether the reverse lights are on or off as I swivel the neutral start switch. I set it so the setpoint is halfway across the span that represents when the reverse lights are on.

Sixto
95 S420
87 300SDL