i dont know if i would do that. now youre drawing unwanted attention to your car and some prick might go up to it and start effin with it. its more fun to drive arund and flash the lights lol
I wouldn't do it for that reason alone, but there's a better reason not to do it. While LED's draw very little power, a full underbody kit is a lot of LED's. Some of the cheap kits that aren't super bright
might not kill your battery in a day, but I bet one of the good kits with really bright LEDs will. Even if they don't kill your battery in a day, your alternator will be working overtime everytime you start the car, as it will be trying to run the car and replenish a low battery. Not only will that eat up alternators, but batteries don't like charging and discharging a lot and you'll eat barreries faster than normal too.
Your best bet is to just wire it up to a switch on constant power so you can turn it on whenever you want, driving or not, car on or not. I just wouldn't recommend leaving them on for long periods of time with the car off on a regular basis.
Whatever you do it is always a good idea to separate your aftermarket electrical accessory circuit from the rest of the car. It's never a good idea to just tap into the fuse box or stock circuits to get your power as it can possibly overload some systems or cause other problems. If you want a constant power source, run a wire directly to the battery and put a fuse on the battery end and a switch for the accessory. If you want to run off a switched power source, run the same constant wire, get a relay, then get your switched power source for the relay from the stock fuse box or wherever. The source for the relay is just a signal, and therefore won't add any strain to whatever circuit you tap into.
We did a relay on our Koup that is run off switched power, and that relay runs our LED driving lights, LED headlight trim, and interior LED's. It will also run our underbody kit when we get it. From that relay, we can run to a number of different switches to turn whatever we want on or off.
I can't tell you how many people blow fuses because they run all their accessories in one wire to a stock fuse for the radio and they're drawing 15 amps with the stuff they added through a 20 amp fuse. They then decide to stick a 30 amp fuse to keep from blowing fuses and burn wires to the point of catching their car on fire because they're pulling too much juice through the system.
If you understand how car fuses work (OEM prong style), 1 side of where the fuse plugs in is power coming in from the underhood power/relay/fuse box, the other side is power going out to whatever circuit that fuse is regulating. If you hook your accessory wire after the fuse, it's still probably pulling too much juice through the system and could cause problems like blowing fuses or worse, but at least it's fused. It's when they hook it to the other side, before the fuse, that things get really risky. Then your accessory power is unregulated and unfused and not only draws too much power through the circuit, but also has a greater chance of burning up your wiring. Running a relayed power circuit will take either of these problems out of the equation.