This may seem really simple to some, but I'm pretty ignorant when it comes to electrical.
I have a standard trailer connector plug with 4 or 5 (too lazy to go out and look) circle plugs in a row, for all the lights on my trailer.

Well, some of the lights work and some dont

Should I just go buy another plug and pull the old one out?

I have no idea on how to troubleshoot this thing, even though I do have a very nice voltmeter

Oh, and I did check the bulbs on the trailer

 

You could check all the wires on the trailer and make sure they all hook up where they need to.
Or you could take it to a trailer place and have them look at it for you because they know what they are doing, my buddy had to do that with his truck because we were to stupid to figure it out.

 

1. check your bulbs
2. check the grounds.

edit: I have found this link helpful in the past:
http://www.etrailer.com/faq-wiring.aspx

 

It is probably due to the fact that you have separate brake lights & turn signals on the truck.
You need this: http://www.wiringproducts.com/contents/en-us/p559.html also get some weather tight butt connectors.
I had the same problem with mine, bought the converter @ autozone for about $20. Took about 5 minutes to install converter & all is well.

 

Ocdropzone thanks for that link. Zing went to my toyota folder for future ref.

 

Chances are that our converter is shot. Best bet is to get a new harness from etrailer if this is the case.