The 1989 Porsche 928 S4 with the automatic transmission uses a Mercedes 722.3 Transmission. I was having problems shifting from drive to reverse. I would actually sometimes turn off the car then put the car in park, turn it on again and then into reverse. The transmission was rebuilt, I replaced the vacuum modulator, governor as well as rebuilt the valve body with a shift kit.
The Porsche Community only has a few transmission gurus. So, I started doing research on Mercedes forums and found out the Neutral Safety Switch was the problem. I could not find replacement instructions on any Porsche forum so I ended up using an article written by a Mercedes mechanic. The process was very simple. I already had the exhaust removed to make space and the car jacked up in the air. I only had to remove 2 bolts and disconnect the shifter cable. I was surprised on how simple and expensive the part was but it obviously worked because it prevented me from going into reverse.