The starter relay stud is close to battery, but it is less so when battery is in trunk, the OP pondering that.. "Close" involves small inductance and resistance. :) The starter stud has a significant transient when the starter solenoid releases. A radio is fairly tolerant, ECUs, results may vary depending on experience of hardware designer. Use of a good scope reveals nasty transients. Even modern cars have transients with load dump up to 6OV. The several feet of cable placing battery in trunk, presents issues.