Imagine I'm the supplying dealer.
In comes a customer who bought a car off me three months ago. He says that the wiring is butchered around the gearbox. The car has been fine up until now. Another garage has changed bits, and the owner himself has modified it in his ownership so it puts out more power. I don't have any recollection of any issues at all, and the car has been fine for months. I'm unlikely to see the customer again, it's been a tight month and tbh I can't see an easy fix so this is going to cost me money. The owner has faffed around with it, personally I quite fancy my chances in court as I reckon I have a good chance of proving that the fault wasn't there on purchase as otherwise it would've started smoking straight away, and I have a good PDI to prove it.
Nope, I'm not taking the car back, but I will look at it for him and potentially charge for repairs.
Is just one scenario. If the car hadn't been modified, and the other garage hadn't already touched it, then yes it's a slam dunk take it back. In this case, it's questionable.