Does the starter relay 12v battery post need 12v?

Actually running an auxillery power distribution unit like that is a good idea.

For the OP my suggestion is to figure out how the power supplies are connected.
If system is using a regular ammeter in the battery line, then when the engine is running, power is going to run backward through the ammeter and fusible link when the amplifier is drawing power.

There's a couple of ways to solve that. One is to route power like one of the heavy duty/fleet options. Another is to add a line direct from the alternator to the battery positive (with a fuse). This will result in the ammeter not showing battery charging, so adding a voltmeter gage will help monitor system.
In either case, an auxillery fusebox or a fuse/relay box can be added in the engine bay.

Diagrams showing similar situations here
https://www.forabodiesonly.com/mopar/threads/alternator-field-wires.459766/post-1972923668replace EFI with stereo.

Another one where someone wanted to add an auxilery box, but also using a Denso alternator with built in voltage regulator.
https://www.forabodiesonly.com/mopar/threads/fusible-link-or-not.458737/post-1972904806