First you need to do a simple check to localize the problem.

With the front wheels SECURELY blocked, jack up the rear of the car and disconnect the shift linkage rod, it doesn't matter which end, just do the one that is easiest to get to.

Then shift the tranmission into park by hand by pushing on the shift selector bracket.

I the tranmission now shifts into park and the pawl holds you need to adjust the linkage rod.

If it doesn't hold then you have a problem with the pawl and/or sprag inside the transmission

Geoff Furr

1985 380 SE