making wiring harness from scratch

May I ask why you are doing this. I have bought both the engine forward and under dash wiring harnesses from year-one. I paid 150.00 for the engine forward and 500.00 for the under dash. The second seems to be costly but I would say well worth it. Both come with all new wiring and factory correct connectors. The under dash comes with a brand new fusebox and firewall connection block. =P~

On the other hand I believe 20 guage wire may be too small. Most of my harnesses use 18, 16 and 14 guage plus some larger ones for the main feed to the amp meter from the engine compartment. 8)