budda,
Wonderful news..and so fast. Good for you. I would not think the horn problem would cause the tail light problem, but I think it indicates the harness install might not be glitch free.
Now that the rig starts, get the charging system working properly. If you can disconnect the taillights like you did the horn, I would do that. You want the charging system isolated so you can prove it works, without any questionable wiring influencing it.
Because you have an external regulator, your alternator might check OK on the bench, but not be wired correctly in the truck to charge.
See what develops on the alternator test, and be sure to disconnect the battery until you resolve more of the problems.
Stuart
