issue bypassing amp on 93 s13 - not enough power

[kablexxx]

hey guys

i was recently given a set of component speakers for the front of my s13. after pulling apart everything and hooking up the wires to test i ran into a problem. the drivers side speaker is getting some power but isnt hitting hard like the other side is. the input wire from the back of the head unit appears to be the issue. i troubleshooted it and its not either speaker, crossover unit, or tweeter. they all work fine on their own.

after reading a bit on here i decided to bypass the amp and go directly from the head unit into the crossover to see if that was the problem. no luck with that either. so i followed the wire to the back of my head unit (pioneer AVH-P5700 in-dash dvd player 50x4).

i'm not too good with wiring which is why i'm here. the blue/white and blue/yellow wires run into a harness that connects into the head unit. between the harness and the head unit the wire is spliced and appears to go from a 16 gauge to a 14 gauge - but what the hell do i know, thats what it said on the connector where the splice was. then that wire runs into the back of the unit.

my questions are:

1. if i want to test it again closer to the unit and cut the wire where is the best point to do this from? (speaker side of harness, between harness and splice, between splice and head unit, or take them all one step at a time)

2. i saw some people suggest running your own wires...should i just go for this and cut the wire closest to the head unit and wire it from there?

3. what gauge wire and what brand should i pick up? i tried a bit of searching but its hard to narrow it down to that given gauge is such a common word.

if the answer to number 2 is yes. how do i know it will work after i run the wire? i'd like to at least test it at some point so i know i can at least splice it at a good point.

pics below - starting from door speaker running up to the back of the head unit - thanks in advance for the help!