Originally Posted by projectRDM

Drilling through the firewall always means using grommets. I didn't think that was noteworthy to mention but good point, stupid people frequent this board too.

Originally Posted by projectRDM

As far as a diagram, think about it. You have say six items that all need a signal from one point. So take all six leads, connect all six together at said common point. You can also just do a 'daisy chain' where you string them together like Christmas tree lights, it really doesn't matter. I prefer to tie it all in at one point so all the original wiring for the gauges or whatever I'm installing isn't cut overly short, if need be it can be moved or installed elsewhere without having to add more wire. On my own car I bought molex plugs and made a separate subharness for the gauges, allowing them to be removed without unplugging each unit individually.

Seriously, it's your car, you have the ability to do it how you would prefer it to look/function. There is no right way to do it as long as the connections are secure, free from potential damage, and nicely installed for a clean look. There's tons of good wiring guides all over the internet if you just want to look at harness builds, solder joints, tie wrap installs, etc.