Trunk Mount Batt. Opinions needed

Draw out a diagram of the car's starting system, that is from body up to battery ground, from battery to disconnect to wherever that goes and on up to starter

Photograph a decent shot so we can read it and post it here. We/ you can measure voltage points along the path and it will show you instantly where the trouble lies, if drop is the problem.

This is no different than a street map. You leave you house. It's a 25mph street. You go down to the through street. It's a 35mph. You end up on highway whatever, a connector. It's 35 out of town, then 45 or 55, and then you get on the freeway, it's 70

All these changes are just exactly like battery cables. Each cable, each connection, each point in the system has RESISTANCE. And you have to measure that change with VOLTAGE under LOAD