In reply #1 of the following is a photo that matches what you posted.
In reply #7 of the following is a wiring diagram that supposedly is the
one for your 'Clifford G4' aftermarket alarm system.
(
How to remove aftermarket alarm from my Miata?)
Here is how I would approach this puzzle:
1) Can I get a color match between the unknown wiring diagram and what's in my vehicle?
2) IF yes to #1, trace the red power input wire back to the fuse. Q: If you pull the fuse does
this disable your car? If not, then this should be an easy removal/clean up.
3) IF the car is disabled in Step #2 when you pull the fuse, then obviously the old security alarm
is still working and not just abandoned in place. Reinstall the fuse, and trace the wiring to the
Ignition disable (and optional starter disable) connections. Remove all traces of the aftermarket
wiring, and repair your vehicle's wiring harness back to new condition.
NOTE: Do yourself a favor, and check that the truck still starts after each individual wiring harness
repair. (!)
Pretty quick you will arrive at the point where pulling the power fuse no longer affects the starting
of your truck. As soon as this happens, now you can relax and prune the rest of it out at your
leisure.
4) Do not be tempted to just leave the rest of it in your GMT400. I've troubleshot several weird
electrical gremlins back to aftermarket alarms like this. (Circuits get cross-connected, batteries
run flat after a couple of days of not driving the car, etc.) Been there, fixed that. (See attached.)
Unlike the thread I shared, please come back with what you found/fixed for the benefit of others
with similar issues.
Best of luck. Happy hunting --