Let me propose what might be an answer but maybe not the answer to why you can't find much. Think about how many people actually want to do whatever it is that you are trying to do. Don't count the people who simply buy the car equipped that way. From the group of people who want to do it - how many will actually put time/energy into it. Many of those people haven't the time nor the resources (money, skills, etc) to approach it in DIY fashion. Some pay the money for aftermarket equipment - others just give up. What you have left are the few who want to do it and actually can do it. Now that still may not be a small number of people but divide that number by the countless variations that exist in sensors. Note also that of all the people who want to do it - only a portion would take the time to document what they've done and make it available for free.
With all of the above in mind you might be lucky enough to own a vehicle that is popular and/or common - in the sense that there are groups or clubs of people among the DIYers. Maybe those groups or clubs might help lead you to what you need.
One popular misconception is that one can do it themselves less expensively - sometimes it can be done but quite often it's not the case, especially with electronics. Even if you don't count your time (which has to be worth something) gathering all of the right stuff (presuming that kits don't exist) can be costly. The PCB, the parts, the enclosure, etc, etc.
This isn't intended to discourage you or anyone. I tend to do lots of things myself - just like many others. There is a lot of satisfaction to be had in making something yourself. I can't make a simple bookshelf for less money than I can buy one - but the one I made means more to me. I am building a small transmitter that will cost me much more than the cost of a new one - but I am doing it for the experience, the education and to prove to myself that I can do it. If those are your reasons let me encourage you to keep going. You might have to take a step back to learn more so you can apply things that are commonly available to the project of interest - beg, borrow, steal. It may not be pretty in the end and not what you'd design if you were a professional but it's yours.
Good luck.