Got a 1989 Fender Strat(USA) with normal 5 position selector switch.  It's all stock except I added a single coil Carvin hummbucker pickup.  Now my selector switch when it's all the way at the ends then nothing comes through the amp.  I'm thinking that some wiring came loose but just wanted other opinions.  

Is your Carvin humbucker a 4-wire or 2-wire version? If it is a 4-wire version, you need to connect two of those wires together to complete the humbuckers. This is if you are keeping it simple and only want to connect two wires to your controls (switches/pots) similar to your single coils. Otherwise, the humbucker will appear as an open circuit to your amp and you will get very little if any output.

Edited to add that if there are 4 wires, you will need to check the Carvin documentation as to which two colored wires are to be connected. Manufacturers don't use the same color scheme like Duncan and Dimarzio.

I have no clue as to 4 wire or 2 wire.  I will say that this guitar has had the same configuration for about 15 years.  It's just done this recently.  

Then I would say that either the switch is worn, or one of the wires came off a terminal on the switch. For worn switches, jiggle or tap the thing to see if you can get some output from the amp. If so, replace the darn thing. I've done this last year on my 86 MIJ.

If that does not work and you checked the obvious such as your volume knob is up, then you probably will have to take off the pickguard and look underneath.

Also, it could be that the two wires I mentioned previously were just twisted together and came undone recently.  I had a Duncan STK-S2 in the bridge postion that did this several years ago.

Quoted: Then I would say that either the switch is worn, or one of the wires came off a terminal on the switch. For worn switches, jiggle or tap the thing to see if you can get some output from the amp. If so, replace the darn thing. I've done this last year on my 86 MIJ. If that does not work and you checked the obvious such as your volume knob is up, then you probably will have to take off the pickguard and look underneath. Also, it could be that the two wires I mentioned previously were just twisted together and came undone recently.  I had a Duncan STK-S2 in the bridge postion that did this several years ago.

I jerked off the selector switch a few times.  Intermittently I can get some response through the amp on the back and rear pickup.  Leads me more to believe something is loose or worn.  

The switch is probably just worn.

You can either replace it, or you can take it out of the pickguard, disassemble it (2 screws), and clean the contact areas with some alcohol, and pry the "fingers" of the switch assembly so that they make more positive contact. Double check that no wires are loose or have come unsoldered.

This is very common.

try squirting some contact cleaner in there...sometimes that helps.  I have an Ibanez like that.  I shot of 2 of that stuff every 6 months or so, and it's good to go.