Don't know about a Civic specifically, but when I installed driving lights
on my vehicle, I just looked for the existing wiring harness, and ran my
wire along side of that.  You may have to use something like a straightened
coat hanger to push it through.

When I put an alarm in my '90 Integra RS, I pulled the left fender
completely off.  There's a nice spot to drill a hole, over by the door
instead of directly through the firewall. (Be sure of what's behind
that you're drilling into!)

I recall there was a wiring harness that also passed through there, but
I could not get another wire through.  It was far too tight.  I put a
little grommet in the hole I drilled, passed the wiring through, and
siliconed the opening before closing up.  Took quite a while to do all
this, but it was a pretty trick installation IMHO.

JM

Like Bob V said, try to push it though along side an existing wire
harness. Rather than using a coat hanger, I use a large nylon tie-wrap
(head cut off) lubed up with silicone lubricant.

passenger side, kick panel, I found a rather large gromet, perhaps for air
conditioning, I pulled that out, ran a boatload of wires through it, then
used expanding foam to seal it back up.

Dan