You can just make a jumper for your starter that jumps from the main battery cable terminal to the S terminal and run your original S wire to the solenoid in the trunk to the S or I terminal. So now you would just have the heavy battery cable from your trunk to the starter, and now that it is jumped to the S terminal on the starter, when it is powered from the solenoid switched in the trunk it will crank.
Just remember that all your 12 Volt power needs to be from the trunk now