You didnt say what 5 speed that you have but a lot of later models have a neutral safety switch that could be used to signal the interlock relay. When the trans was in neutral, the relay would retract the pin; when in gear, the relay would engage the pin. Of course, you would have to build in a delay so that the pin didnt retract while you were shifting.

If it were an automatic, it would be easy to just use a park switch.


mike in tucson