Thanks for everyone's responses. I did end up going to Brownwood Village Golf Carts and they did sell me the brackets for $42.
My neighbor opened up his dash on both of his Yamahas (installed by Village Golf Carts) and they have this voltage regulator between the switched power and the speakers:
DC 5.5~32V to 1~27V 5A Buck Converter/Adjustable Voltage Regulator/Car Adapter/Power Supply Module/Driver Module
What would be the purpose of this? Just to make sure the voltage going into the speakers would be around 12 v without the spikes, etc? Doesn't seem like the speakers would be that sensitive since they're designed to be connected directly to a motorcycle battery.