A very small brush and a steady hand!
If you were painting the firewall body color, you could "paint" the molded in wires with colored Sharpies, but since the firewall will be black, the only way to highlight those molded-in wires is by painting them or maybe using a colored gel pen (might take several coats from the gel pen to get opaque coverage over black, though).
I've seen sets of 6 different colored gel pens on sale at Office Max and Office Depot. Might be worth a shot.