Auto Transmission shift cable adjustment/ removal

[missilehowie]

Recently my boxter will not start. All the lights come on but there is no starter. If I depress the brake and move the shifter out of park and back in, it will start sometimes. This has happened three times each is getting worse, once I was told that my back up lights were on while I was driving. I checked the shift cable and found that it is not very tight. I have tried to turn the cable nut at the transmission shift lever, but it won't move. I don't want to break something, but I do not see any other way this can work. Am I missing something, or should I just turn it harder.

[RFM]

The cable adjustment mechanism is located under the shifter, if you are not familiar with the system, lack on tools (adjustment tool), is it cheaper to have it done at the end, it takes only 15 minutes for someone with knowledge.