[Richard]

You can wire the speakers two ways, series or parallel. Car speakers are typically rated at 4 ohms.

Series you wire the + to - from one speaker to the other then connect the + from one speaker and - from the other to amp or head unit. This will double the load amp or head unit will see. Two 4 ohm speakers will become an 8 ohm load, typically reducing power by half.

Parallel is simply connecting both + and both - to the amp or head unit. This will reduce load by half. Two 4 ohm speakers will become a 2 ohm load. Typically power is increased sometimes by two. Will have to make sure the amp or head unit can handle a 2 ohm load.
Lighter load can cause damage, more load just reduces sound output.