a site that discusses plug-types that you may encounter, color codes,
troubleshooting, and more. Pretty good.
The Champion Trailer site offers some "technical" articles that are worth a read. They cover most things.
The Old Burb (Chevy Suburban) Club has some wiring tips.
The Classic Trailers site has some schematics that could prove helpful - they cover different systems, from four to seven wire setups.
The Marine Mechanic site deals with largely with four-wire hookups which are not suitable for "real" trailers, but many details apply equally in either case, and they do discuss the separate tail/turnsignal light issue.