I found the following description on another forum:

door speaker wiring

Front LEFT Driver Door Spkr POSitive (+) color is.......... O/LG (orange/lime green)<?XML:NAMESPACE PREFIX = O /><O:P style="MARGIN: 0px"> </O:P>
<O:P style="MARGIN: 0px"></O:P>
<O:P style="MARGIN: 0px"></O:P>Front LEFT Driver Door Spkr NEGative (-) color is......... LB/W (light blue/white) <O:P style="MARGIN: 0px"> </O:P>
<O:P style="MARGIN: 0px"></O:P>
<O:P style="MARGIN: 0px"></O:P>Front RIGHT Passenger Door Spkr POSitive (+) color is ...... W/LG (white/lime green)<O:P style="MARGIN: 0px"> </O:P>
<O:P style="MARGIN: 0px"></O:P>
<O:P style="MARGIN: 0px"></O:P>Front RIGHT Passenger Door Spkr NEGative (-) color is ..... DG/O (dark green/orange)

It seems to be Ford's own standard?

I replaced the factory speakers (cheap chinese dual cone) with a pair of MBQuart two-way dual in line speakers and had to find out, that Ford uses a special plug, with no marking for plus/minus on the cable/plug/speaker.

The factory wiring by the way has a ridiculous gauge. But I didn't have the nerves and time to replace it immediately.