Let’s say you’re looking to modify your car. Maybe it’s down on power, maybe it could use a nice spoiler or neon lights. Well in order to get more power, you’ll need to modify the engine. If you want to put a spoiler on the car, you’ll have to drill holes into the bodywork. If you want neon lights you’ll have to tamper with the car’s electronics. None of these are good ideas.
If you’re gonna do any of that to your car, what you’re doing is essentially destroying the integrity of the car. Manufacturers build cars to particular specs, everything in the car is designed to work together. If you put an outsider in, the car no longer functions as it’s supposed to. This is why it’s bad to modify, or, add things to your car that don’t belong. What’s the harm in drilling a couple of holes and fitting a spoiler? It will disrupt the aerodynamics, damage the structural integrity of the trunk lid, and possibly allow water to come in and damage it. Maybe the trunk lid material isn’t strong enough to support the weight, and so it bends under the aerodynamic force.
You should not modify your car, unless you plan on keeping it forever. Typical modification means changing the car to be something it wasn’t designed to be, I.e. putting a turbocharger on an engine that wasn’t built for it. Engines turbocharged from the factory have the correct pistons, are tuned properly, and if a non-turbocharged engine is forced to perform beyond its limits, things will break and then the engine will be worthless. That said modifying a car, unless the stars align and you find that correct buyer, can significantly lower the value of the car because some people don’t want to deal with a car that’s no longer built properly and might break all the time.
Another reason to stay with factory parts is sometimes they were built better than aftermarket ones. That said there are a few things on modern cars that aren’t very strong. For instance, radiators on Honda S2000s are made of plastic and are known to explode. Likewise the thermostat housing on E36 BMWs is made of plastic, which can also explode. Failing cooling systems are bad because if the car overheats the heads can warp while overheating, and when they cool back down might no longer be aligned with the engine block. When that happens, coolant mixes with oil and the engine won’t run properly. Sometimes car manufacturers use plastic because it saves manufacturing costs, and in that sense modifying something that breaks isn’t necessarily bad practice. After all, it’s improving on something that was already there on the car. The trouble with modifying is when you try to add something that wasn’t there in the first place, that doesn’t belong.
Tech improvements like a GPS or subwoofers aren’t always a good idea either. Both of those require tampering with the electric system of the car, and even those have been tuned specifically for the car. Cars have batteries, alternators and harnesses that are designed to work with a specific amount of electrical power. Adding a GPS draws more than the car is capable of and can cause a lot of electrical problems where the car might drain more power than it can recharge, and then you’ll be dealing with a dead car that won't start up on its own power.
Subwoofers are dangerous for the same reason. They need amplifiers that can work with them, and those coupled with the subwoofer is detrimental to the electrical system. They simply can draw too much power and cause the car’s charging system to not work properly, and before you know it you’re jump starting your car every day just to get to work.
If you plan on modifying your car, choose to replace a broken part with something of better quality. For instance, better quality brake pads and rotors are good modifications to make, because you’re not adding something onto the car that doesn’t belong, but rather enhancing an area of the car that needs improvement. Or if your car has rubber suspension bushings and they start to deteriorate (as they often do), you can replace them with polyurethane which performs better.
Remember to have all of your modifications done by a shop. They will do it properly, and if something doesn’t fit they will let you know because they don’t want to be liable for any damages to the car. If you have a mechanic do the work you’ll also have the added assurance that your car will work as intended. Also you’ll know your car is legal after the modifications.
The bottom line of modifications is, you shouldn’t be doing them on your car if they don’t belong there. If you add something to the car that shouldn’t be there, you’ll be messing with a perfect structure that operates exactly as it was supposed to. Only modify a part if it’s an improvement on an existing part. That way you’ll know that it fits properly (hopefully) and isn’t going to screw anything up.