Get another installer. He's wrong.

See my site for my install complete with wiring chart for the adapter plug you can use to connect your harness to your new head unit.


Click

plug shown goes to your car, wires to the head unit (color code is documented on my site)

If you remove your head unit later, the car is unchanged, just unplug and you're back to stock.

If your installer can't handle this they should not be installing stereos - this is entry-level stuff.