MSD Wiring w/ Trunk-Mounted Battery

MSD boxes are very forgiving with supply voltage, not sensitive, will work with as low as 5v & deliver full voltage with 9-18v.
As far as the direct grd wire to the box, I would connect the thick black wire to the engine...or....grd strap if the body/frame is used for bat [-]. The MSD box only knows voltage, not where it comes from....