Cars made before a certain date make the parallel connection between the doors
and floors in the Door Sill.
The Alpine kit official install procedure describe where this connection
is in detail.
https://www.google.com/url?sa=t&rct=...1qePRZCIR7aEzz
see page 10
If you have the 4 pin connection the Technic jumper in his kits take care of the split.
It's important you understand what's happening speakers mostly only have two terminal
inputs except for dual coil subs. The fact there are 4 wires is one set of two is and and Input
from the Head Unit or Amp. The other set of wires run's back to the doors.
If you are trying to add better speakers in a Base system with no amp there is a good chance
it won't work very well or at all.