Verify with a test LIGHT which has a SHARP POINT to scrape through corrosion or pierce insulation. I used to have lots of V notched broom handle segments cut up for holding wire while I stabbed it.
Cut the wire AHEAD of the stab hole. If the conductor is green, brown or purple instead of new penny copper color, be prepared to scrape wire strands shiny with a utility knife blade. Scraping the tarnish off the copper wire strands is more than worth "the bother".
I love the Chinese color code sensibility. Blue and yellow, Brown and black, purple and gray. And 24 gauge conductor for 12 volt 8 amp power.
TIP: If after doing your best and the lamp still does not seem to work right, blame chassis negative. If the negative is corroded or compromised, it'll turn your turn signals / running lights into a veritable nuthouse. This is a MAJOR fault cause.