Turn signal issue resulted in checkingof wires. Unannounced by the seller an autostart feature was installed (Kind ofPoorly) not ran to power source but to a existing wire via stripping splicing.
Wiring can be a little frustrating, but if you do it one at a time using the solder and shrink wrap method it should look good and function as if that unit was never there. Judging from your post it sounds like you have had it with this car, wanting to scrap it, I've seen many cars wind up in the scrap heap due to failed circuits and people just plain feed up with the mess, but hang in there, your lucky your just dealing with one problem.
Try to restore the original wiring by splicing in a patch, seal it with shrink tubing. Then run the seller's add-on back to the fuse panel, you can add circuits with an "add-a-circuit" kit. Don't be afraid to peel kick or trim panels off to get to wiring, it may look daunting but just stick to it, you'll get it done.
What am I looking at cost wise? And do auto parts stores carry color coded wires I need?
All the big ones do (Autozone, Advance, O'Rielly), Walmart also. Auto stores are not the cheapest places to get that stuff, so long as you're not buying a lot of it there it'll do in a pinch. I'd get a soldering iron & solder from someplace like Radio Shack tho. Search YT vids on in-line splicing tips, use shrink tubing to cover the splice and make it watertight, just be sure to slide the shrink tubing on before you splice the wires. If you want to keep the autostart, you can run it on its own circuit, with a MINI add-a-circuit. Those are great for adding new circuits to the fuse box, if you don't have any open spaces. If the autostart needs more current than a typical 15-20 amp one, you can run it back the battery post and put an in-line fuse in it. Cost wise is minimal, it's mostly labor, which is why it's better to tackle it yourself than farm it out.
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