My quick silver 3000 throttle for my 1989 Searay mec cruiser is giving me trouble. It only starts if I wriggle it

Your shifter cable is most likely almost out of adjustment. What I think your issue is .... Your shifter cable has stretched so neutral on your shifter isn't actually neutral on your motor. There is a safety system built into every boat motor that will not allow the motor to start while in gear ( think early cartoons of the guy flying over the back when he fires up his boat) if that switch is even slightly out of position then your motor will not even send a signal to the starter. Try resetting the cable to its proper position. Next check that ground wire on the motor and make sure it's tight.... The cables maybe moving the ground wire around.