High level input wiring?

I'm wiring an amp using the high level input in my leased car.the information said that "Only positive wires were used. don't connect speaker negatives"

What exactly does this mean?

I've installed many systems into cars but have never used high level inputs before.

I understand what it says but why could they put negative hookups on it if you are not supposed to use it?

It means exactly what it says: Connect only the positive wire from a left and right speaker and don't connect a wire to a negative terminal. Speakers have two terminals, a negative and a positive. When a speaker gets a positive signal on its positive side and a negative signal on its negative side, the speaker cone will advance. When the polarity is reversed, the speaker moves rearward. If you do not know which terminal is which, you may be able to use this knowledge to learn. Simply take a one 1/2 volt battery AA, AAA, etc. and connect two short wires to the ends and touch them to the speaker terminals. If the cone moves forward out, you have the positive battery terminal on the positive speaker wire. If it moves rearward, the positive battery is on the negative speaker terminal.

Another technique is to look carefully at the widths of the terminals. Most speakers except Bose use the wider terminal as positive.

The amp seemingly gets its signal ground through the power supply ground.

I have been a professional mobile electronics installer for 25 years.