In short, the installation of fog lights (PTF) is a useful thing for someone, but to consider them a panacea is a big mistake. In addition, it is useful for any home-made man to remember this.
First: if the manufacturer of your car at one time did not provide a fog lamp on it (as many examples as you like), then it seems impossible to install them yourself. The traffic rules, as has long been known, prohibit making any changes to the design of a car without the knowledge of the plant. A "smart" traffic inspector understands this as he needs to - and easily damages for any trifle. Even for a non-standard bolt - a "secret" of fastening of a wheel. And here - the lights! The violation is egregious, because in the text of modern traffic rules there is not a word about additional ones. Hence the interpretation of interested traffic cops: “Not allowed - then forbidden!” When buying foglights, be prepared “in which case” to fight back from extortion.
Second: if you put the PTF - it’s only as it was stipulated in the old rules - below the headlights, and horizontally - not wider than them. Nothing new has been invented in this area. In order for the road to be visible through the fog, the headlights should illuminate its canvas, and not the muddy haze in front of the driver's eyes. (Having turned on the high beam in the fog, everyone will understand that this is not good - you can’t see anyone behind such “milk”.) Therefore, if someone is punished for foglights on the roof, then it’s right. And how else to admonish stupid?
So, the places for the PTF are defined, the fastenings, brackets, etc. are made. It is up to the electrician. How to connect foglights to the on-board network?
Let's start with the choice of wire cross-section - it should be large enough so that the power of powerful lamps is not burdened by large losses. For orientation, raise the hood and look at the wires serving the standard headlights. For fog, you need about the same, but not a smaller section.
The reference “plus” for PTF is better to stretch directly from the battery - this power cable will minimize losses. And directly into this wire to embed a fuse of about 15 A. But that's not all. In order not to rape the weak contacts of the PTF switch with a large current, an unloading relay is introduced into their power supply circuit, which we spoke about in previous conversations. To operate (turn on) the relay, a relatively small cross-section wire is suitable (there are a lot of similar ones under the hood) - you can connect it, for example, as in scheme 1 - to the power supply of the “dimensions” lamps. This is done on many production cars.
And now - about the installation of an additional brake light (“understudy”). Someone is limited to one lamp, another and two are few. But the car is not a Christmas tree. The main thing is to try to avoid blunders. Photo 2 shows what the “effect” of installing an additional lamp behind tinted (!) Glass: in the afternoon - no good.
If there are no questions about the scheme, let's start connecting. The power of the lamps is not too large - here you can do without an unloading relay. But there are some difficulties - for example, wires from understudies to regular brake lights will be needed (diagram in Fig. 2).
American cars deserve a special talk in this regard, in which the same incandescent threads perform the functions of “stops” and direction indicators. On "Americans" it is best to extend additional wires (according to the scheme in Fig. 3) from the "frog" to the understudies. If you connect the understudies to the “stop” lamps, then if the additional lamps are unsuccessfully placed, those traveling behind you, ceasing to understand anything, risk losing their peace of mind. Yes, and you hardly need a dangerous mess with the signals. (Incidentally, for this reason, it is not easy to connect a Russian trailer to an American car.)