Positive(+) is red in your application , negative(-) is to ground and black. Note, wires on your 'sickle can be pink or other colors; +, and, green is often -.
Put switch , Or, Diode in + lead.
Use a 10 amp Schottkey diode (50 cents on Ebay), the banded side to the alternator output, unbanded to + on battery. Split the wire from alt to battery and insert as I said. Connect + on amp to the the banded side of diode. This way amp will not get power until alternator turning ( bike running.) Beats some switch U have to remember to hit.
Electronics tech 40 years, repaired/customized/.upgraded electrical on cages and 'sickles. Honda, Yammer, Harley.
Electricity is Easy. Clean. Clutches on a FWD are Hard, Greasy.