no real need for a harness...and no real splicing either...

aside from jumpering the park/nuetral switch so the car will start...what you need to after that is wire your reverse lights

all you need is to take a meter to the auto harness.
plug it into the middle grey plug and find which one is the constant (green/blue on the fusebox side) and which one is the reverse (white/green on the box side) by testing em on a voltage meter and cut everything else off the plug.
plug it back into the middle grey plug off the fuse box, and run the two wires down to the furthest front plug (labeled reverse light sensor or something along those lines)...stick the car in reverse and have some look at the reverse lights, then plug it up to the tranny plug until they come on.