Most brand stereos have an adaptor for Peugeot models (Sony, Kenwood seem to anyway). The adaptor costs a bit, but is very tidy.
If you don't want to do that, a simple way to work out which wires go to which speaker is to connect a AA battery across speaker wires at the stereo end & listen for crackle. The speaker wires are usually next to each other in the harness so don't confuse with the positive V+ which is usually red, & switching V+, yellow.
You will require a voltmeter for tracing the constant power V+ & ignition switching V+.
Battery ground is black.
Get all your speakers with the same polarity too otherwise one side will feed the other & will be out of phase + no or bugger all bass