Killswitch installation?
 

So I'm trying to install a killswitch on my truck which is a 1990 pickup 3vze. I was able to find the blue wire that's in the circuit opening relay but my concern is can i just T-tap the blue wire that's coming out of the circuit opening relay and call it a day? Or do I have to locate the specific blue wire that splits and leads towards the fuel pump?. Please help I don't want to mess up my truck for trying to do the work myself. I super new to messing with wires and photos would be very much appreciated. I placed a red circle on the part where I see the wire split for a better understanding sorry in advance I'm a noob.
https://cimg8.ibsrv.net/gimg/www.yot...5af8220be.jpeg

Given that the light Blue wire that goes off the page goes to the Data Link Connector, it may be safely disregarded.

What do you mean "T-Tap" a wire?
If you want to put a kill switch in the line from the COR to the fuel pump, and yes, the light blue wire is the one to use, you need a SPST switch, two contacts, one way the switch is open, the other way it's closed, in series with the wire.
IOW, cut the wire, run a wire from one side of the cut, to one side of the switch. Then from the other side of the switch to the other side of the cut you made in the light blue wire. No "T" involved.
Everything is in series, and a "T" implies a parallel set up. Just be certain you either have the cut in the light blue wire either before I14 junction, or in the wire going to the fuel pump after the I14 junction point. If you cut the wire going to the Data Link Connector and put the switch in that, it will have no effect whatsoever. Better to use the wire before the I14 junction, where the wire to the test block splits off.

Does that help at all?
Pat☺

https://cimg4.ibsrv.net/gimg/www.yot...1961156002.jpg

if by T-tap you mean to use one of these, i would recommend against it.
I am not sure how many connections I have fixed over the years because a circuit has been fubar'ed by this type of connector.
might seem like a good idea but in my opinion by the very nature of the device you are cutting several strands of wire and limiting current flow.

Honestly it has been very helpful. I would of loved photos but I think I have it figured out now. Thanks a bunch you the best. I do have one last thing to ask. What type of wire gauge is recommended for this type of work? Or working on this truck in general please

Honestly I was doing some research about this and read the same you speak of. What would be the best method? Instead of using T-tap connectors.

My bad I brought up the term T-tap I'm such a noob at this and learning alot along the way and all of this is mind blowing 😅

Those T-Taps are widely known as vampire splices in the electronics/electrics world. I don't know any of us recommend using them. Instead of a decent splice, you just trash the wire you want to splice into, and after a while, chances are it will start getting intermittent. And THOSE are the biggest PITA to trouble shoot, trust me.

Having said all that, using a splice like that will only make a parallel path for current flow through your switch, not a series path, which is what you need. You want to be able to open up the circuit to the fuel pump. Essentially, you're pulling the wire going back to the fuel pump off from the COR, so no current can flow to the fuel pump. That's why I said, cut the wire, put in a splice on one side that runs TO your switch, and a splice on the other side of the cut to the other side of your switch. THAT is a "series" circuit. The current goes from the COR, through your switch, and back to the rest of the fuel pump circuit. Thus, when your switch is ON, the current can flow from the COR, through your switch, and then to the fuel pump. When your switch is OFF, no way for current to get to the fuel pump.
No fuel pump, no engine running.

Using the T-Tap, you're adding your switch "Beside" the rest of the circuit. In other words, the current will flow to the fuel pump AND through your switch. No way for your switch to stop the fuel pump from running. On or Off, the current can still make it to the fuel pump. Hence, a "parallel" circuit. NOT what you want.

Let me see if I can make a drawing of each kind of circuit. I am a horrible artiste, though. I can't draw a straight line with a ruler...
Series circuit. Use your imagination. The COR would go about where the "I" is, your switch would be where the switch is, ignore the resistor, and the inductor, marked "L" would be the fuel pump. The battery is, well, the battery :D
When your switch is closed, or ON, the fuel pump can operate normally. When your switch is OFF, no current can get to the pump, so it won't function. Thus no fuel, no engine.
Note: Current flows negative to positive, actually, not positive to negative as shown. Not important here, just a little detail that bugs me.
https://cimg7.ibsrv.net/gimg/www.yot...4c3b7f9321.gif
Below is a parallel circuit. Again, use your imagination here.
Imagine the upper X is your switch, the bottom one is the fuel pump. You can see that whether your switch is on, or off, the pump gets current, and thus, functions normally. No good for a kill switch :)
https://cimg3.ibsrv.net/gimg/www.yot...4c83f5e28c.jpg


I would use a 10 ga or 12 ga wire to do this. It seems like most of the "general purpose" wires in these trucks are 12 ga or 14 ga wires. If you use 10-12 ga wires, you're pretty sure to have wire that can carry the required current flow. Remember, when speaking of wires like these, the larger the number the smaller the wire. Say, for example, a 12 ga wire is smaller, and carries less current, than a 10 ga wire.When in doubt, say the wire LOOKS like a 12 ga, but you're not sure, use the next size larger. In this example a 10 ga wire. Just to be safe.
Make sure you have a fuse in the circuit, in series with your switch. Again, just to be safe.

Does all that babbling, and the lousy diagrams, help at all?
Pat☺