If he has an external voltage regulator, and it's not getting enough amperage, that could very well cause his flashing headlight problem. I know from very painful personal experiences. Thanks to the horrible customer support of Excessive Amperage, I had to figure out how to fix this problem myself, with my 300 amp alt on my Explorer.
I had to run 2 10 or 15 amp 5 pin relays to my external voltage regulator, and I had to use 2 8 gauge wires and stack them on top of each other for both power and ground, it was quite a stupid and poor design, honestly. Why does a thing that supposedly draws 10-15 amps of current need essentially like 30 amps of current? A lot of these ext voltage regulators are the same model, and it seems like many companies should actually explain how to properly install them, which a lot don't seem to. There should be a manual that they send with the alt and ext regulator. The external voltage regulators, many that I've seen, leave a lot of room for improvement.
Like my alt was a super good performing alt, when the damn regulator worked properly. Took me and my shop friend who is a certified car alarm installer (very difficult work) something like a couple of months to actually figure out the regulator powering was the problem. Ugh, dude, it was so frustrating. I was 19 when I figured this problem out. I was just lucky enough to have experienced people around me and have access to a shop full of testing equipment and tools.
