This was happening on my 97 SL, the shifter bushing was missing preventing the trans from going fully into park. ( the bushing on the shift side not the trans side )

There is a small cable that starts at the trans that gives in park confirmation. From parts diagrams, on the SL it goes to the parking brake release then to the ignition / steering column lock.

Columns on these cars lock only when the key is removed, the park confirmation prevents the key from turning that last little bit to release the key. All of this prevents someone from turning off the key and locking the column while in motion. Other cars use a button on the column, of the column shift is interlocked to the steering lock.