you are very welcome.
As for your query about wire guage..
Just make sure you join them wires properly with (Idealy) solder and heatshrink, good crimps at a push but not by twisting together and wrapping in selotape, you may laugh, I have seen it done!
some will say it makes no...
the wires you need are the 2 pairs in the middle of the speaker block, just snip them off at the end that goes into the CAR wiring, NOT at the headunit end...
The standard ISO plug speaker wiring. (connections viewed from cable side of connector)
TAB
Right Rear(1) Right Front(1) Left...
to kep the existing rear wires simply cut and join the front wires and let the iso adaptor work its magic on the rears!
as to why there is no ready made kit for this....
how long u want the wires, what guage, what colour the list is endless, and you would still have to route the wires, which...
check your earth connection to the head unit.. when i last did one i used the earth wire from the iso adaptor, cutting it at the MICRA end, then soldered a ring on it and fitted it under one of the radio mounting screws!