I wired the inverter plus and minus to the battery directly.
Fuse close to the battery.
I also grounded the inverter case to the shell of the car using the wingnut/thumbscrew on the case supplied, and grounded the PSU, and Motherboard and tft to the same place.
I fitted a relay into the main power leads for the inverter. I wired this top the remote line of the headunit, but you could use the acc line of your ignition switch, or a switch 12v source from the fusebox. This allows the inverter to be completley hidden, yet switchable without having to reach the switch on the inverter, disconnect the battery, or fit a mnual switch in line, that you can forget to flick.
My 300watt gets warm, but is fan cooled. Has been under the rear seat through summer and winter, I did cut the carpet to route the fan into the cabin for cooler air, but I don't know if that is necessary.